JATC and his friend Giraffe are currently in their room, solving some problems. Giraffe has written on the board an array a_1, a_2, ..., a_n of integers, such that 1 ≤ a_1 < a_2 < … < a_n ≤ 10^3, and then went to the bathroom.

JATC decided to prank his friend by erasing some consecutive elements in the array. Since he doesn't want for the prank to go too far, he will only erase in a way, such that Giraffe can still restore the array using the information from the remaining elements. Because Giraffe has created the array, he's also aware that it's an increasing array and all the elements are integers in the range [1, 10^3].

JATC wonders what is the greatest number of elements he can erase?

Input

The first line of the input contains a single integer n (1 ≤ n ≤ 100) — the number of elements in the array.

The second line of the input contains n integers a_i (1 ≤ a_1<a_2<...<a_n ≤ 10^3) — the array written by Giraffe.

Output

Print a single integer — the maximum number of consecutive elements in the array that JATC can erase.

If it is impossible to erase even a single element, print 0.

Examples

Input

6
1 3 4 5 6 9


Output

2

Input

3
998 999 1000


Output

2

Input

5
1 2 3 4 5


Output

4

Note

In the first example, JATC can erase the third and fourth elements, leaving the array [1, 3, \\_, \\_, 6, 9]. As you can see, there is only one way to fill in the blanks.

In the second example, JATC can erase the second and the third elements. The array will become [998, \\_, \\_]. Because all the elements are less than or equal to 1000, the array is still can be restored. Note, that he can't erase the first 2 elements.

In the third example, JATC can erase the first 4 elements. Since all the elements are greater than or equal to 1, Giraffe can still restore the array. Note, that he can't erase the last 4 elements.


import java.util.*;
import java.lang.*;
import java.io.*;
import java.math.*;


public class file{


// *********************************************BEGINS*********************************************
public static void main(String[] args) throws IOException{


// Working program using BufferedReader 
	 //BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        // int test=Integer.parseInt(br.readLine());


PrintWriter pt=new PrintWriter(System.out);
Reader scn=new Reader();
//int test=scn.nextInt();
int test= 1;


while(test-->0){

	int n=scn.nextInt();
	int[] arr=new int[n];
	for(int i=0;i<n;i++) {
		arr[i]=scn.nextInt();
		}
	if(n==1) {
		pt.println("0");
		
	}else if(n==2){
		
		if(arr[1]-arr[0]==1 && (arr[1] ==2 || arr[0]==999)) {
			pt.println("1");
		}else {
			pt.println("0");
		}
	}else {
		
		int c=0;
		int ans=0;
		for(int i=1;i<n;i++) {
			if(arr[i]-arr[i-1] == 1) {
				if(i==1 && arr[i-1]==1) {
					c=2;
				}else {
					c++;
					
				}
			}else {
				if(ans < c-1) {
					ans=c-1;
			
				}
				c=0;
			}
			
		}
		
		if(arr[n-1] == 1000) {
			if(ans < c) {
				ans=c;
			}
		}else {
			if(ans  < c-1) {
				ans=c-1;
			}
		}
		
		pt.println(ans);
         
	}
}

pt.close();

}
// *********************************************ENDED*********************************************
static int sqrt(int x){
               return (int)Math.sqrt(x);
       }

static int ceil(int x, int y) {
		return (x % y == 0 ? x / y : (x / y + 1));
	}
 
static long ceil(long x, long y) {
		return (x % y == 0 ? x / y : (x / y + 1));
	}

static boolean isarrequal(int[] array1,int[] array2){
        Object[] arr1 = {array1};   
        Object[] arr2 = {array2}; 
        if (Arrays.deepEquals(arr1, arr2)) 
           return true;
        else
            return false;
}

static boolean isarrequal(int[][] array1,int[][] array2){
        Object[] arr1 = {array1};   
        Object[] arr2 = {array2}; 
        if (Arrays.deepEquals(arr1, arr2)) 
           return true;
        else
            return false;
}

static int abs(int x) {
		return Math.abs(x);
	}
 
static long abs(long x) {
		return Math.abs(x);
	}

static int max(int x, int y) {
		return Math.max(x, y);
	}
 
static int min(int x, int y) {
		return Math.min(x, y);
	}
 
static long max(long x, long y) {
		return Math.max(x, y);
	}
 
static long min(long x, long y) {
		return Math.min(x, y);
	}

static int min(int a[]) {
		int x = 1_000_000_00_9;
		for (int i = 0; i < a.length; i++)
			x = min(x, a[i]);
		return x;
	}
 
static int max(int a[]) {
		int x = -1_000_000_00_9;
		for (int i = 0; i < a.length; i++)
			x = max(x, a[i]);
		return x;
	}
 
static long min(long a[]) {
		long x = (long) 3e18;
		for (int i = 0; i < a.length; i++)
			x = min(x, a[i]);
		return x;
	}
 
static long max(long a[]) {
		long x = -(long) 3e18;
		for (int i = 0; i < a.length; i++)
			x = max(x, a[i]);
		return x;
	}

static int power(int x, int y) {
		int res = 1;
		while (y > 0) {
			if (y % 2 == 1)
				res = (res * x);
			y >>= 1;
			x = (x * x);
		}
		return res;
	}

static long power(long x, long y) {
		long res = 1;
		while (y > 0) {
			if (y % 2 == 1)
				res = (res * x);
			y >>= 1;
			x = (x * x);
		}
		return res;
	}


   static boolean isPrime(long n)
    {
 
        // Check if number is less than
        // equal to 1
        if (n <= 1)
            return false;
 
        // Check if number is 2
        else if (n == 2)
            return true;
 
        // Check if n is a multiple of 2
        else if (n % 2 == 0)
            return false;
 
        // If not, then just check the odds
        for (int i = 3; i <= Math.sqrt(n); i += 2) 
        {
            if (n % i == 0)
                return false;
        }
        return true;
    }

	
//static boolean isprime(long n) 
//{ 
    // Converting long to BigInteger 
   // BigInteger b = new BigInteger(String.valueOf(n)); 

   // return b.isProbablePrime(1); 
//} 

static void merge(int arr[], int l, int m, int r)
    {
        // Find sizes of two subarrays to be merged
        int n1 = m - l + 1;
        int n2 = r - m;
 
        /* Create temp arrays */
        int L[] = new int [n1];
        int R[] = new int [n2];
 
        /*Copy data to temp arrays*/
        for (int i = 0; i < n1; ++i)
            L[i] = arr[l + i];
        for (int j = 0; j < n2; ++j)
            R[j] = arr[m + 1 + j];
 
 
        /* Merge the temp arrays */
 
        // Initial indexes of first and second subarrays
        int i = 0, j = 0;
 
        // Initial index of merged subarry array
        int k = l;
        while (i < n1 && j < n2)
        {
            if (L[i] <= R[j])
            {
                arr[k] = L[i];
                i++;
            }
            else
            {
                arr[k] = R[j];
                j++;
            }
            k++;
        }
 
        /* Copy remaining elements of L[] if any */
        while (i < n1)
        {
            arr[k] = L[i];
            i++;
            k++;
        }
 
        /* Copy remaining elements of R[] if any */
        while (j < n2)
        {
            arr[k] = R[j];
            j++;
            k++;
        }
    }
 
    // Main function that sorts arr[l..r] using
    // merge()
    static void sort(int arr[], int l, int r)
    {
        if (l < r)
        {
            // Find the middle point
            int m = (l + r) / 2;
 
            // Sort first and second halves
            sort(arr, l, m);
            sort(arr , m + 1, r);
 
            // Merge the sorted halves
            merge(arr, l, m, r);
        }
    }

static class Reader 
    { 
        final private int BUFFER_SIZE = 1 << 16; 
        private DataInputStream din; 
        private byte[] buffer; 
        private int bufferPointer, bytesRead; 
  

public Reader() 
        { 
            din = new DataInputStream(System.in); 
            buffer = new byte[BUFFER_SIZE]; 
            bufferPointer = bytesRead = 0; 
        }


public Reader(String file_name) throws IOException 
        { 
            din = new DataInputStream(new FileInputStream(file_name)); 
            buffer = new byte[BUFFER_SIZE]; 
            bufferPointer = bytesRead = 0; 
        }  

public String nextLine() throws IOException 
        { 
            byte[] buf = new byte[64]; // line length 
            int cnt = 0, c; 
            while ((c = read()) != -1) 
            { 
                if (c == '\n') 
                    break; 
                buf[cnt++] = (byte) c; 
            } 
            return new String(buf, 0, cnt); 
        } 


 public int nextInt() throws IOException 
        { 
            int ret = 0; 
            byte c = read(); 
            while (c <= ' ') 
                c = read(); 
            boolean neg = (c == '-'); 
            if (neg) 
                c = read(); 
            do
            { 
                ret = ret * 10 + c - '0'; 
            }  while ((c = read()) >= '0' && c <= '9'); 
  
            if (neg) 
                return -ret; 
            return ret; 
        } 


 public long nextLong() throws IOException 
        { 
            long ret = 0; 
            byte c = read(); 
            while (c <= ' ') 
                c = read(); 
            boolean neg = (c == '-'); 
            if (neg) 
                c = read(); 
            do { 
                ret = ret * 10 + c - '0'; 
            } 
            while ((c = read()) >= '0' && c <= '9'); 
            if (neg) 
                return -ret; 
            return ret; 
        } 


public double nextDouble() throws IOException 
        { 
            double ret = 0, div = 1; 
            byte c = read(); 
            while (c <= ' ') 
                c = read(); 
            boolean neg = (c == '-'); 
            if (neg) 
                c = read(); 
  
            do { 
                ret = ret * 10 + c - '0'; 
            } 
            while ((c = read()) >= '0' && c <= '9'); 
  
            if (c == '.') 
            { 
                while ((c = read()) >= '0' && c <= '9') 
                { 
                    ret += (c - '0') / (div *= 10); 
                } 
            } 
  
            if (neg) 
                return -ret; 
            return ret; 
        } 

private void fillBuffer() throws IOException 
        { 
            bytesRead = din.read(buffer, bufferPointer = 0, BUFFER_SIZE); 
            if (bytesRead == -1) 
                buffer[0] = -1; 
        } 

private byte read() throws IOException 
        { 
            if (bufferPointer == bytesRead) 
                fillBuffer(); 
            return buffer[bufferPointer++]; 
        } 


public void close() throws IOException 
        { 
            if (din == null) 
                return; 
            din.close(); 
        } 
    }
  
}