#include <iostream>

using lint = long long;

void solve() {
    lint a, b, c;
    std::cin >> a >> b >> c;

    auto p = c * 3600,
         q = a * 60 - b;

    lint ans;
    if (q > 0) {
        ans = (p + q - 1) / q;
    } else {
        ans = -1;
    }

    std::cout << ans << std::endl;
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}