#include<bits/stdc++.h>
using namespace std;
using ll = long long;

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

#define FOR(i,a,b) for(ll i=(a);i<(b);++i)
#define ALL(v) (v).begin(), (v).end()
#define p(s) cout<<(s)<<endl
#define p2(s, t) cout << (s) << " " << (t) << endl
#define br() p("")
#define pn(s) cout << (#s) << " " << (s) << endl

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

    // input
    ll a,b,c;cin>>a>>b>>c;
    
    ll merit = 60*a - b;
    if(merit<=0){
        p(-1);
        return 0;
    }

    ll bunsi = 3600*c;
    ll bunbo = 60*a-b;

    ll x;
    if(bunsi % bunbo ==0){
        x = bunsi/bunbo;
    }else{
        x = bunsi/bunbo + 1;
    }
    p(x);
    
    return 0;
}