A few years ago Sajjad left his school and register to another one due to security reasons. Now he wishes to find Amir, one of his schoolmates and good friends.

There are n schools numerated from 1 to n. One can travel between each pair of them, to do so, he needs to buy a ticket. The ticker between schools i and j costs <image> and can be used multiple times. Help Sajjad to find the minimum cost he needs to pay for tickets to visit all schools. He can start and finish in any school.

Input

The first line contains a single integer n (1 ≤ n ≤ 105) — the number of schools.

Output

Print single integer: the minimum cost of tickets needed to visit all schools.

Examples

Input

2


Output

0


Input

10


Output

4

Note

In the first example we can buy a ticket between the schools that costs <image>.

 
    		import java.awt.List;
    		import java.io.*;
    		import java.util.*;

    		
    		
    		/*
    		 * 
    		 * daiict
    		 *
    		 * 
    		 * 
    		 */
    		
    		
    		public class prac {
    			FastScanner in;
    			PrintWriter out;
    		
    			static ArrayList[] adj;
    			
    			static int n;
    			static int m;
    			
    			static int[] dp1;
    			static int[] dp2;
    			static int color[];
    			static int visited[];
    			static long[] arr;
    			static long[] brr;
    			static long ans;
    			static int array[];
    			static int pref[];
    			static int tree[];
    			static int maxval = 32;
    		
    			static int sum = 0;
    		//for dfs do not delete
    			static int dir[][] ={{1,0},{0,1},{0,-1},{-1,0},{1,1},{1,-1},{-1,1},{-1,-1}};
    			
    		
    			static long[] seg;
    			static long count = 1;
    			static long mod = (long)1e9 + 7;
    	
    			static int parent[];
    			static int size[];
    			
    			static class Pair implements Comparable<Pair>
    		    {
    				long x;int y;
    				
    		        Pair(long l, int m) {
    		          
    		        	this.x = l;
    		            this.y = m;
    		          
    		        }
    		        
    		        public int compareTo(Pair p) {
    		            if (this.x == p.x) {
    		                return (int)(this.y-p.y);
    		            }
    		            return (int)(this.x - p.x);
    		        }
    		        
    		        public String toString(){
    		        	return x + " " + y;
    		        }
    		        
    		        public boolean equals(Object o){
    		        	if(o instanceof Pair){
    		        		Pair a = (Pair)o;
    		        		
    		        		return this.x == a.x;
    		        	}
    		        	
    		        	return false;
    		        }
    		        
    		        public int hashCode(){
    		        	return new Long(x).hashCode()*31 + new Long(y).hashCode();
    		        }
    		    
    		    }
    		    
    			
    			public static int gcd(int a, int b) {
    				if (b == 0) return a;
    				return gcd(b, a%b);
    			}
    	
    			void build(int n) {  // build the tree
    				  for (int i = n - 1; i > 0; --i) seg[i] = Math.max(seg[i<<1],seg[i<<1|1]);
    				}
     
    			public static void modify(int p, int i,long val,int n) {  // set value at position p
    					if(i == 1){
    						long temp = val;
    						val = seg[p+n] - arr[p] + val;
    						arr[p] = temp;
    					}else{
    						val = Math.min(val, p) + arr[p];
    						brr[p] = val;
    					}
    					
    				  for (seg[p += n] = val; p > 1; p >>= 1) seg[p>>1] = Math.max(seg[p],seg[p^1]);
    				}
     
    				long query(int l, int r,int n) {  // max on interval [l, r)
    				  long res = Integer.MIN_VALUE;
    				  for (l += n, r += n; l < r; l >>= 1, r >>= 1) {
    				    if ((l&1) == 1) res = (int)Math.max(res,seg[l++]);
    				    if ((r&1) == 1) res = (int)Math.max(res,seg[--r]);
    				  }
    				  return res;
    				}
    				
    			public static void update(int x,int val){
    				while(x<=maxval){
    					tree[x] += val;
    					x = (x&-x);
    				}
    			}
    			
    			public static int read(int idx){
    				int sum = 0;
    				
    				while(idx>= 0){
    					sum += tree[idx];
    					idx -= (idx&(-idx));
    				}
    				
    				return sum;
    			}
    			
    			public static int find(int i){
    				if(parent[i] != i){
    					return find(parent[i]);
    				}
    				
    				return i;
    			}
    			
    			public static void union(int a,int b){
    				int x = find(a);
    				int y = find(b);
    				
    				if(x!=y){
    					if(size[x] > size[y]){
    						size[x] += size[y];
    						parent[y] = x;
    					}else{
    						size[y] += size[x];
    						parent[x] = y;
    					}
    				}
    			}
    			
    		    void solve() throws NumberFormatException, IOException {
    		 
    		    	//Scanner in = new Scanner(System.in);
    		    	
    		    	long n = in.nextLong();
    		    	
    		    	out.println(n - n/2 - 1);
    		    	
    		    //	out.println();
    		    
    		    }
    		    
    		    public static long invcount(int arr[]){
    		    	if(arr.length<2){
    		    		return 0;
    		    	}
    		    	
    		    	int mid = (arr.length + 1)/2;
    		    	
    		    	int left[] = Arrays.copyOfRange(arr, 0,mid);
    		    	int right[] = Arrays.copyOfRange(arr, mid, arr.length);
    		    	
    		    	return invcount(left)+invcount(right)+merge(arr,left,right);
    		    	
    		    }
    		    
    		    public static long merge(int arr[],int left[],int right[]){
    		    	long count = 0;
    		    	
    		    	int i = 0;
    		    	int j = 0;
    		    	
    		    	while(i<left.length || j < right.length){
    		    		
    		    		if(i == left.length){
    		    			arr[i+j] = right[j];
    		    			j++;
    		    		}else if(j == right.length){
    		    			arr[j+i] = left[i];
    		    			i++;
    		    		}else{
    		    			if(left[i] <= right[j]){
    		    				arr[i+j] = left[i];
    		    				i++;
    		    			}else{
    		    				arr[i+j] = right[j];
    		    				j++;
    		    				count += left.length-i;
    		    			}
    		    		}
    		    		
    		    	}
    		    	
    		    	return count;
    		    }
    		    
    		  
    		    
    		    /**
    		     * 
    		     * @param div
    		     * @param left
    		     * @param right
    		     * @param value
    		     * @return the smallest index i which a[i] >= value in range [left, right) if exists, otherwise return -1
    		     */
    		    int ff(int[] div, int left, int right, int value) {
    		        int low = left;
    		        int high = right;
    		        
    		        if(value > div[right-1]){
    		        	return left;
    		        }
    		        
    		        while (low != high) {
    		            int mid = (low + high) / 2;
    		            if (div[mid] < value) {
    		                low = mid + 1;
    		            } else {
    		                high = mid;
    		            }
    		        }
    		        if (low == right) {
    		            return low;
    		        }
    		        return low;
    		    } 
    		   
    		    /**
    		     * 
    		     * @param div
    		     * @param left
    		     * @param right
    		     * @param value
    		     * @return greatest index i which a[i] <= value in range [left, right) if exists, otherwise return a number out of [left, right)
    		     */
    		    int ffff(int[] div, int left, int right, int value) {
    		        
    		    	if(value < div[left]){
    		    		return right-1;
    		    	}
    		    	
    		    	while (left != right) {
    		            int mid = (left + right) >> 1;
    		            if (div[mid] <= value) {
    		                left = mid + 1;
    		            } else {
    		                right = mid;
    		            }
    		        }
    		        if (left < 0) {
    		            left *= -1;
    		        }
    		        return --left;
    		    }
	    
    		   
    		  
    		
    		    public static String reverse(String s){
    		    	
    		    	String ans = "";
    		    	
    		    	for(int i = s.length()-1;i>=0;i--){
    		    		ans += s.charAt(i);
    		    	}
    		    	
    		    	return ans;
    		    	
    		    }
    			
    			
    		    
    		 
    		    public static long sum(long x){
    		    	return x<10?x:x%10 + sum(x/10);
    		    }
    			
    			public static long gcd(long x, long y) {
    				if (x == 0)
    					return y;
    				else
    					return gcd( y % x,x);
    			}
    			static long inv(long x , long mod)
    			{
    				long r,y;
    				for(r=1,y=mod-2;y!=0;x=x*x%mod,y>>=1)
    				{
    					if((y&1)==1)
    						r=r*x%mod;
    				
    				}
    				return r;
    			}
    			
    			public static long pow(long x,long y,long n){
    				if(y==0)
    					return 1%n;
    				if(y%2==0){
    					long z=pow(x,y/2,n);
    					return (z*z)%n;
    				}
    				return ((x%n)*pow(x,y-1,n))%n;
    			}
    			
    			
    			
    			
    			public static boolean isPrime(int n) {
    				// Corner cases
    				if (n <= 1)
    					return false;
    				if (n <= 3)
    					return true;
    		 
    				// This is checked so that we can skip 
    				// middle five numbers in below loop
    				if (n % 2 == 0 || n % 3 == 0)
    					return false;
    		 
    				for (int i = 5; i * i <= n; i = i + 6)
    					if (n % i == 0 || n % (i + 2) == 0)
    						return false;
    		 
    				return true;
    			}
    			
    		
    			void run() throws NumberFormatException, IOException {
    				in = new FastScanner();
    				out = new PrintWriter(System.out);
    		
    				solve();
    		
    				out.close();
    			}
    		
    			class FastScanner {
    				BufferedReader br;
    				StringTokenizer st;
    		
    				public FastScanner() {
    					br = new BufferedReader(new InputStreamReader(System.in));
    				}
    		
    				String next() {
    					while (st == null || !st.hasMoreTokens()) {
    						try {
    							st = new StringTokenizer(in.br.readLine());
    						} catch (IOException e) {
    							// TODO Auto-generated catch block
    							e.printStackTrace();
    						}
    					}
    					return st.nextToken();
    				}
    		
    				int nextInt() {
    					return Integer.parseInt(next());
    				}
    		
    				long nextLong() {
    					return Long.parseLong(next());
    				}
    		
    				double nextDouble() {
    					return Double.parseDouble(next());
    				}
    			}
    			
    			
    			
    			
    		 
    		
    			public static void main(String[] args) throws NumberFormatException, IOException {
    				new prac().run();
    			}
    		}
    		class Node{
    			String s;
    			int b;
    			
    		}
    		
    		class MyComp implements Comparator<String>{
    		    
    			int c = 0;
    			
    			MyComp(int d){
    				c = d;
    			}
    			
    			@Override
    			public int compare(String a,String b) {
    				
    				// < == -1
    				
    				String arr[] = a.split(" ");
    				String brr[] = b.split(" ");
    				
    				return arr[c].compareTo(brr[c]);
    				
    				
    			}

    			
    		}
    		
    		class MyComp2 implements Comparator<Node>{
    		    
    		
    			
    			@Override
    			public int compare(Node a,Node b) {
    				
    				// < == -1

    				
    				if(a.b > b.b){
    					return -1;
    				}else{
    					return 1;
    				}
    				
    				
    			}

    			
    		}
