#include<stdio.h>
#include<iostream>
#include<algorithm>
#include<vector>
#include<string>
#include<utility>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<functional>
#include<math.h>
#include<random>
#include <bitset>
#include <cassert>
#include <complex>
using namespace std;
#define N (1000000000+7)
//#define N 998244353
#define INF 1e16
typedef long long ll;
typedef pair<int,int> P;
const int inf = 1e8;




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