using System;
using System.Collections.Generic;
class Program
{
	static void Main(string[] args)
	{
		string[]str = Console.ReadLine().Split();
		long A = long.Parse(str[0])*60;
		long B = long.Parse(str[1]);
		long C = long.Parse(str[2])*3600;
		long X = -1;
		if(A>B){
			X = (C-1) / (A-B)+1;
		}
		Console.WriteLine(X);
	}
}