using System.Numerics;

public class Program
{
    public static void Main()
    {

        //BigInteger num = BigInteger.Parse(Console.ReadLine() ?? string.Empty);
        //int num = int.Parse(Console.ReadLine() ?? string.Empty);
        string[] moji = (Console.ReadLine() ?? string.Empty).Trim().Split(' ');
        //string str = Console.ReadLine() ?? string.Empty;

        BigInteger a = BigInteger.Parse(moji[0])*60;
        BigInteger b = BigInteger.Parse(moji[1]);
        BigInteger c = BigInteger.Parse(moji[2])*60*60;

        

        BigInteger count = 1;
        BigInteger pro = c + (b * count);
        BigInteger hand = a * count;
        if(a/60 == 1)
        {
            Console.WriteLine(-1);
            return;
        }
        while (pro > hand)
        {
            pro = c + (b * count);
            hand = a * count;
            count++;
        }
        Console.WriteLine(count-1);

    }
}