#include<bits/stdc++.h>
typedef long long int ll;
typedef unsigned long long int ull;
#define BIG_NUM 2000000000
#define HUGE_NUM 99999999999999999
#define MOD 1000000007
#define EPS 0.000000001
using namespace std;



int main(){

	int F;
	scanf("%d",&F);

	printf("%d\n",(F-30)/2);

	return 0;
}

