#include <bits/stdc++.h>
const int INF = 1e9;
const int MOD = 1e9+7;
const long long LINF = 1e18;
#define dump(x)  cout << 'x' << ' = ' << (x) << ` `;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) for(int i=0;i<(n);++i)
#define REPR(i,n) for(int i=n;i>=0;i--)
#define FOREACH(x,a) for(auto& (x) : (a) )
typedef long long ll;
using namespace std;
int main(int argc, char const *argv[]) {
    ll a, b, c;
    cin >> a >> b >> c;
    a *= 60;
    c *= 3600;
    if (a < b) cout << -1 << endl;
    cout << (c+(a-b)-1)/(a-b) << endl;
    return 0;
}