using System;
public class Program
{
    static void Main()
    {
        int[] b = Array.ConvertAll(Console.ReadLine().Split(' '), s => int.Parse(s));
        int a = 10;
        for (int i = 0; i < b.Length; i++)
        {
            if (b[i] != i + 1)
            {
                a = i + 1;
                break;
            }
        }
        Console.WriteLine(a);
    }
}