#include <iostream>
typedef long long ll;
using namespace std;

int main(void) {
    // A * 60 * X >= C * 60 * 60 + B * X
    // (A * 60 - B) * X >= 3600 * C
    ll A,B,C;
    cin>>A>>B>>C;
    ll X;
    if (A * 60 - B > 0) {
        X = (3600 * C % (A * 60 - B) != 0) ? 3600 * C / (A * 60 - B) + 1 : 3600 * C / (A * 60 - B);
        cout << X << endl;
    } else {
        cout << "-1" << endl;
    }
    return 0;
}