//g++ -std=c++11 -Wall -O2 -o main.exe main.cpp
//g++ -std=c++14 -Wall -O2 -o main.exe main.cpp

#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <iostream>
#include <iomanip>
#include <string>
#include <vector>
#include <map>
#include <queue>
#include <stack>
#include <tuple>
#include <functional>
#include <algorithm>
#include <complex>
#include <numeric>

using namespace std;

#define spc " "
#define MOD 1000000007

typedef long long ll;
typedef long double ld;
typedef pair<int, int> p_ii;
typedef tuple<int, int, int, int> tup;

bool tupComp(tup &t1, tup &t2){
    return get<2>(t1) > get<2>(t2);
}

//------------------------------------------------------------------------------

int main(){
    ll a, b, c;
    cin >> a >> b >> c;
    a *= 60; c *= 3600;

    ll ans = max((ll)-1, (c + (a - b - 1)) / (a - b));

    cout << ans << endl;
    return 0;
}