#include<iostream>
#include<vector>
#include<algorithm>
#include<functional>
#include<queue>
#include<stack>
#include<set>
#include<map>
#include<unordered_map>
#include<climits>
#include<cstdlib>
#include<cmath>
#include<string>
#include<iomanip>

using namespace std;

#define INF 1 << 29
#define LL long long int

LL const MOD = 1000000007;

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

    LL a,b,c;

    cin >> a >> b >> c;

    if(60*a > b){
        cout << (3600*c + 60*a - b - 1)/(60*a - b) << endl;
    }else{
        cout << -1 << endl;
    }
    
    return 0;
}