#include<iostream>
#include<algorithm>
#include<vector>
#include<cstdio>
#include<cmath>
using namespace std;
using ll = long long;

int main(){
    ll a,b,c,x=0;
    cin >> a >> b >> c;
    if(a*60<=b){
        cout << -1 << endl;
        return 0;
    }
    c*=3600;
    a*=60;
    x = c/(a-b)-1;
    while( c > (a-b)*x ) x++;
    cout << x << endl;
    return 0;
}