#include<bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i=a;i<b;i++)

int A;
//-----------------------------------------------------------------
int main() {
	cin >> A;
	
	int X = A - 7;
	if (X < 8)
		cout << -1 << endl;
	else
		cout << X << endl;
}