using System.Numerics;

public class Program
{
    public static void Main()
    {

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

        BigInteger mi = BigInteger.Parse(str[0]);
        BigInteger sec = BigInteger.Parse(str[1]);
        BigInteger time = BigInteger.Parse(str[2]);

        if(mi == 1)
        {
            Console.WriteLine(-1);
            return;
        }

        int count = 0;

        while(count < 114516)
        {
            count++;
            BigInteger hand = (60 * mi) * count;
            BigInteger pro = ((time * 60)*60) + (count * sec);
            if(hand >= pro)
            {
                break;
            }
        }

        Console.WriteLine(count);
    }

}