#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

int main(){
    ll a, b, c, d;
  	cin >> a >> b >> c;
    c = c * 3600;
    a = a * 60;
    d = a - b;
    cout << ( ( (c+d-1)/d < 0 ) ? -1 : (c+d-1)/d )  << endl; 
    return 0;
}