#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
using ll =  long long ;
using P = pair<int,int> ;
using pll = pair<ll,ll>;
const int INF = 1e9;
const ll LINF = 1e18;
const int MOD = 1000000007;

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