In Japan, temperature is usually expressed using the Celsius (℃) scale. In America, they used the Fahrenheit (℉) scale instead. $20$ degrees Celsius is roughly equal to $68$ degrees Fahrenheit. A phrase such as "Today’s temperature is $68$ degrees" is commonly encountered while you are in America.

A value in Fahrenheit can be converted to Celsius by first subtracting $32$ and then multiplying by $\frac{5}{9}$. A simplified method may be used to produce a rough estimate: first subtract $30$ and then divide by $2$. Using the latter method, $68$ Fahrenheit is converted to $19$ Centigrade, i.e., $\frac{(68-30)}{2}$.

Make a program to convert Fahrenheit to Celsius using the simplified method: $C = \frac{F - 30}{2}$.



Input

The input is given in the following format.


$F$


The input line provides a temperature in Fahrenheit $F$ ($30 \leq F \leq 100$), which is an integer divisible by $2$.

Output

Output the converted Celsius temperature in a line.

Examples

Input

68


Output

19


Input

50


Output

10
#include <bits/stdc++.h>
 
#define rep(i,n) for(int i=0;i<(n);i++)
#define reps(i,n) for(int i=1;i<=(n);i++)
#define lol long long
#define SUM(n) ((n)+1)*(n)/2 //1〜nまでの総和を求める式
#define mp make_pair
#define fi first
#define se second
#define pu push_back
#define SYOU(x) fixed<<setprecision(x+1) //小数点桁数を指定する
#define abs(x,y) max(x,y)-min(x,y)
#define all(v) v.begin(),v.end()
#define UPDigit(a,b) (a+b-1)/b //小数点切り上げ
const int INF = 0x3f3f3f3f;
const long long LINF = 0x3f3f3f3f3f3f3f3fLL;
const int MOD=int(1e9)+7; 
using namespace std;
using pii = pair<lol, lol>;
typedef vector<int> vit;

//最大公約数
lol gcd(lol x, lol y){
  if(x < y) swap(x, y);
  lol r = x % y;
  while(r != 0){
    x = y;
    y = r;
    r = x % y;
  }
  return y;
}
//最小公倍数
lol lcm(lol x, lol y){
  lol a = x * y;
  return (a / gcd(x, y));
}

signed main(void){
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  int f;
  cin >> f;
  cout << (f - 30) / 2 << '\n';
  return 0;
}

