Find factorial of a number

Write a program to find factorial of a number. Input has to be given at load time.

#include<stdio.h>
#include<stdlib.h>
unsigned long long int factorial(int);
int main(int argc,char**argv)
{
    unsigned long long int fact;
    int n;
    if(argc != 2)
    {
        printf("Invalid Input\nUsage:./a.out positive number\n");
        return 0;
    }
    n = atoi(argv[1]);
    if(n < 0)
    {
        printf("Invalid input\n");
        return 0;
    }
    fact = factorial(n);
    printf("factorial of %d is %llu\n",n,fact);
    return 0;
}
unsigned long long int factorial(int n)
{
    unsigned long long int f = 1;
    for(;n;n--)
    {
        f = f * n;
    }
    return f;
}

Output:

./a.out 5
factorial of 5 is 120

./a.out -4 
Invalid input