#include<iostream>
#include<iomanip>
#include<string>
#include<set>
#include<vector>
#include<utility>
#include<queue>
#include<algorithm>
#include<functional>
using namespace std;
#define LL long long
#define MOD 100000
#define inf 1999999999

int f;

int main() {
	cin >> f;
	cout << (f - 30) / 2 << endl;
	

	return 0;
}

