#include <bits/stdc++.h>

using namespace std;
#define pb push_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define rep(i,n) for(int i=0;i<(n);i++)
constexpr int MOD = 1000000007;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
constexpr int dx[] = {1, 0, -1, 0, 1, 1, -1, -1};
constexpr int dy[] = {0, -1, 0, 1, 1, -1, -1, 1};

template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec){os << "["; for (const auto &v : vec) {os << v << ","; } os << "]"; return os; }
template <typename T, typename U> ostream &operator<<(ostream &os, const pair<T, U> &p){os << "(" << p.first << ", " << p.second << ")"; return os;}

void solve() {
    ll A, B, C;
    cin >> A >> B >> C;
    A *= 60;
    C *= 3600;
    // A * X >= C + B * X
    // A * X - B * X >= C
    // X >= C / (A - B)
    if (A <= B) {
        cout << -1 << endl;
    } else {
        ll X = (C + (A - B - 1)) / (A - B);
        if (A * X >= C + B * X) {
            cout << X << endl;
        } else {
            cout << -1 << endl;
        }
    }
}

int main() {
    std::cin.tie(0);
    std::ios::sync_with_stdio(false);
    cout.setf(ios::fixed);
    cout.precision(16);
    solve();
    return 0;
}