.net 5, MPI factorial

Factorial C# MPI.net .cs

using System;
using System.Diagnostics;
using System.Numerics;

MPI.Environment.Run(ref args, comm =>
{
    if (comm.Rank == 0)
    {
        BigInteger x = BigInteger.Parse(args[0]);
        BigInteger res = 1;

        Stopwatch watch = new();
        watch.Start();
        for (int i = 1; i < comm.Size; i++)
            comm.Send(new Pair(((i - 1) * x / (comm.Size - 1)) + 1, i * x / (comm.Size - 1)), i, 0);


        for (int i = 1; i < comm.Size; i++)
            res *= comm.Receive<BigInteger>(i, 0);
        watch.Stop();

        Console.WriteLine($"{x}! = {res} | {watch.Elapsed.TotalMilliseconds}(ms)");
    }
    else
    {
        Pair res = comm.Receive<Pair>(0, 0);
        comm.Send(factorial(res.A, res.B), 0, 0);
    }
});

static BigInteger factorial(BigInteger a, BigInteger b)
{
    BigInteger res = a;
    while (a < b) res *= ++a;
    return res * b;
}

[Serializable]
class Pair
{
    public BigInteger A { get; set; }
    public BigInteger B { get; set; }

    public Pair(BigInteger a, BigInteger b)
    {
        A = a;
        B = b;
    }
}