Working version

NMul.java

import java.util.Scanner;
import java.math.*;

public class NMul {
    public static void main(String[] args) {
        BigInteger sum;
        long n;

        Scanner sc = new Scanner(System.in);
        while (sc.hasNext()) {
            n = sc.nextLong();

            sum = BigInteger.valueOf(1);

            for (long i = 1; i <= n; i++) {
                sum = sum.multiply(BigInteger.valueOf(i));
            }

            System.out.println(sum);
        }
    }  
}