receiving the function from user defined without passing any value

#include<stdio.h>

int main (){

    

    int sum();

    int a,s;

    s = sum();

    printf("%d",s);

    return 0;

}

int sum()

{

    int x,y,z;

    printf("\n enter the three values ");

    scanf("%d %d %d",&x,&y,&z);

    return(x+y+z);

    

}