#include <bits/stdc++.h>
using namespace std;

int main() {
    int64_t a, b, c;
    cin >> a >> b >> c;
    if (a <= b) {
        cout << -1 << endl;
        return 0;
    }
    cout << (3600 * c + a - b - 1) / (a - b) << endl;
    return 0;
}