#include<bits/stdc++.h>
#define rep(i,a,b) for(int i=int(a);i<int(b);++i)
using namespace std;
typedef long long ll;
int INF = (1LL << 30) - 1;
int MOD = 1e9+7;
main(){
    ll A,B,C;
    cin >> A >> B >> C;
    cout << max(-1LL, (3600 * C + 60 * A - B - 1) / (60 * A - B) ) << endl;
}