Decrementing a number in c as an argument

Note: I am a beginner to the world of C language.
I was trying to write a program that calculates the sum of N natural number.
Here it is:

#include<stdio.h>

int sum_of_N_with_recursion(int N);

int main()
{
    int n,result;
    do
    {
        printf("enter a natural number : ");
        scanf("%d",&n);
        if(n<0)
        {
            printf("%d is not a natural numbern",n);
        }
    }while(n<0);
    result = sum_of_N_with_recursion(n);
    printf("sum of the first %d natural numbers is %dn",n,result);    

    return 0;
}


int sum_of_N_with_recursion(int N)
{
    if(N==0)
    {
        return 0;
    }
    return  N + sum_of_N_with_recursion(N--);
}

What is my problem?

When I tried to decrement N in the sum_of_N_number() like this sum_of_N_number(N--) but it doesn’t work. Why?

Example: sum_of_N_number(3) should equal to 6 but I got 3!!

return  N + sum_of_N_with_recursion(N--);

return  N + sum_of_N_with_recursion(N-1);