using System;
using System.IO;
using System.Linq;

string[] lines = File.ReadAllLines("/var/data/data.txt");

int multiplication = Convert.ToInt32(lines.First()) * Convert.ToInt32(lines.Last());

using (StreamWriter writer = new StreamWriter("/var/result/result.txt"))
{
    writer.WriteLine(multiplication);
}

Console.WriteLine("Произведение успешно записано в файл! Результат: " + multiplication);