Go

package main

import "fmt"

func main() {
	fmt.Println("Hello, World!")
}

package main

import (
	"fmt"
	"log"
)

func factorial(n int) int {
	result := 1
	for i:=1; i<=n; i++ {
		result *= i
	}
    return result
}

func main() {

	fmt.Println("enter a number:")
	var number int
	_, err := fmt.Scanln(&number)
	if err != nil {
		log.Fatal("Invalid input. Please enter a valid non-negative integer.")
	}
	if number < 0 {
		log.Fatal("Invalid input. Please enter a non-negative integer.")
	}

	fmt.Println(factorial(number))
}