#include <stdio.h>
int main()
{
        int n, i;
        unsigned long long factorial = 1;
        printf("Enter an integer: ");
        scanf("%d",&n);
        // show error if the user enters a negative integer
        if (n < 0)
            printf("Error! Factorial of a negative number doesn't exist.");
        else
        {
            for(i=1; i<=n; ++i)
            {
                factorial *= i;              // factorial = factorial*i;
            }
            printf("Factorial of %d = %llu", n, factorial);
        }
        return 0;
}