#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <stack>
#include <queue>
#include <list>
#include <set>
#include <map>
#include <numeric>
#include <regex>
#include <tuple>
#include <iomanip>
#include <cmath>
using namespace std;

typedef long long ll;
typedef pair<int, int> P;
#define MOD 1000000007 // 10^9 + 7
#define INF 1000000000 // 10^9
#define LLINF 1LL<<60


int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	ll A, B, C; cin >> A >> B >> C;
	A *= 60;
	C *= 3600;
	ll ans;
	if (A < B)ans = -1;
	else if (C % (A - B) == 0) ans = C / (A - B);
	else ans = C / (A - B) + 1;

	cout << ans << endl;

	return 0;
}