fct2.go

package main

import (
	"fmt"
	"io/ioutil"
	"math/big"
	"os"
	"strconv"
	"time"
)

func main() {
	n, err := strconv.ParseInt(os.Args[1], 10, 64)
	if err != nil {
		panic(err)
	}
	r := big.NewInt(1)
	start := time.Now()
	for i := int64(2); i <= n; i++ {
		r.Mul(r, big.NewInt(i))
	}
	fmt.Println("Elapsed:", time.Since(start))
	fmt.Println(r)
	ioutil.WriteFile("result.txt", []byte(r.String()), 0644)
}