Why this code gives negative values for some input values??

Question

import java.util.Scanner;
public class product {
public static void main (String [] args){
System.out.println("Enter a number = ") ;
Scanner sc = new Scanner(System.in);
int n=sc.nextInt();
int i=1;
long mul=1;
while(i<=n){
mul=mul*i;
i++;
}
System.out.println(mul);
}

}
input=21
output=-4249290049419214848
why??