How to use function find the sum of two numbers

#include <stdio.h>
int sum(int x, int y); 
int main(void)
{
    int a = 10, b = 20, k;
    k = sum(a, b); /*Function call*/
    printf("%d\n", k);
    k = sum(4, 5); /*Function call*/
    printf("%d\n", k);
    k = sum(a + b, b * 2); /*function call*/
    printf("%d\n", k);
    return 0;
}
int sum(int x, int y)
{
    int s;
    s = x + y;
    return s;
}