#include <bits/stdc++.h>
#include <cstdio>
#include <string>

using namespace std;

#define INF 100000000
#define YJ 1145141919
#define INF_INT_MAX 2147483647
#define INF_LL_MAX 9223372036854775807
#define EPS 1e-10
#define MOD 1000000007
#define Pi acos(-1)
#define LL long long
#define ULL unsigned long long
#define LD long double

#define int long long

signed main()
{
    int A, B, C;
    cin >> A >> B >> C;

    if(A*60 <= B) {
        cout << -1 << endl;
        return 0;
    }

    int l = 0, r = 114514191981036;
    while(r-l > 1) {
        int mid = (l+r)/2;
        
        int a = mid * A * 60;
        int b = C*3600 + mid * B;
        if(a >= b) {
            r = mid;
        } else {
            l = mid;
        }
    }

    cout << r << endl;

    // int diff = A*60 - B;
    // int c = C*3600/diff;
    // if(C%diff != 0) {
    //     c++;
    // }

    // cout << c << endl;

    return 0;
}