#include <algorithm>
#include <array>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iomanip>
#include <iostream>
#include <numeric>
#include <optional>
#include <queue>
#include <set>
#include <sstream>
#include <unordered_map>
#include <unordered_set>
#include <vector>

using namespace std;

int main() {
    int64_t a, b, c;
    cin >> a >> b >> c;
    a *= 60;
    c *= 3600;
    int64_t d = a - b, ans;
    if (d <= 0)
        ans = -1;
    else
        ans = (c + d - 1) / d;
    cout << ans << endl;
    return 0;
}