using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Gos
{
    class Program
    {
        static void Main(string[] args)
        {
            var firstFileText = File.ReadAllLines("first.txt");
            var secondFileText = File.ReadAllLines("second.txt");
            using (var sw = new StreamWriter("first.txt", false))
            {
                for (int i = 0; i < firstFileText.Length; i++)
                {
                    var line = i + firstFileText[i];
                    if (secondFileText.Length > i)
                        line += secondFileText[i];
                    sw.WriteLine(line);
                }
            }
        }
    }
}