How to use sum in C language

#include<stdio.h>
int sum(int x, int y);   /*Function declaration*/
int main()
{
    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)  /*Function definition*/
{
    int s;
    s=x+y;
    return s;
}