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

typedef long long ll;
typedef pair<int, int> P;

#define For(i, a, b) for(int i = (a); i < (b); i++)
#define Rep(i, n) For(i, 0, (n))
#define Rrep(i, n) for(int i = (n - 1); i >= 0; i--)
#define Itr(i, c) for(typeof(c.begin()) i = c.begin(); i != c.end(); i++)
#define pb push_back

const int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
const int MAX = 510000;
const int INF = 999999999;
const int MOD = 1000000007;

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