#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <queue>
#include <cstdio>
#include <ctime>
#include <assert.h>
#include <chrono>
#include <random>
#include <numeric>
#include <set>
using namespace std;
typedef long long int ll;
using ull = unsigned long long;

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    ll a,b,c; cin >> a >> b >> c;
    if(60*a<=b){
        printf("-1\n");
    }
    else{
        a*=60;
        c*=3600;
        printf("%lld\n",(c+a-b-1)/(a-b));
    }
}