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;



        long A = long.Parse(moji[0]);
        long B = long.Parse(moji[1]);
        long C = long.Parse(moji[2]);
        A *= 60;
        C = C * 60 * 60;

        if (A == (C+B))
        {
            Console.WriteLine(1);
            return;
        }

        if (A < B)//(A < C)
        {
            Console.WriteLine(-1);
            return;
        }

        // A > C
        long count = 0;
        BigInteger pro = C+(B*1);
        BigInteger hand = A * 1;

        while(pro > hand)
        {
            count++;
            pro = C + (B * count);
            hand = A * count;
        }
        Console.WriteLine(count);

    }
}