#include<stdio.h>
#include<math.h>
#define ll long long int

ll sum(int n)
{
	if(n == 1){
        	return 1;
    	}else{
        	return ((ll)pow(n, n) + sum(n - 1));
    	}
}

void main()
{
	int n = 3;
	printf("%d", sum(n));
}

-------------------------


At the very beginning the ll ( small L L ) is defined as long long int
which signifies that this function is capable of handling a big number
upto the limit of long long int.

n == 1 is the breakout condtion for the function sum,
in which point function stops recursing.

If the input is greater than 1, function rturns the n'th power of number n
along with a recursion to itself where input is n - 1.

Example:

For input of 3

sum(3)

=> 3^3 + sum(2) => 2^2 + sum(1) => 1
=> 27 + 4 + 1
= 32