#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    ll a,b,c;
    cin >> a >> b >> c;

    a *= 60;
    c *= 3600;

    if(b >= a){
        cout << -1 << endl;
        return 0;
    }

    ll ans = (c - 1) / (a - b) + 1;

    cout << ans << endl;
    return 0;
}