#include <bits/stdc++.h>
typedef long long ll;
#define ALL(a) a, a+sizeof(a)/sizeof(a[0])
using namespace std;
ll gcd(ll n, ll m) {
	if (m == 0) return n;
	return gcd(m, n % m);
}

ll lcm(ll n, ll m) {
	return m / gcd(n, m) * n;
}

int main(void) {
	int f;
	cin >> f;
	cout << (f-30)/2 << endl;
	return 0;
}
