char s1[] = "521";
char s2[] = "1624";
char ris[100];  /*result should go here*/
int i;

sum(s1, s2, ris);

printf("n");

for(i = 0; i < 10; i++){
    printf("%c", ris[i]);
}

printf("n");

return EXIT_SUCCESS;

if(s1 == '' && s2 == ''){
    return 0;
}
else if (remainder == 1){
    ris = ris + 49;
    remainder = 0;
    return auxSum (s1, s2, *is, remainder);
}

else if(s1 + s2  > 57){
    remainder = 1;
    ris = ris + (s1 + s2) - 10;
    return auxSum (s1, s2, ris, remainder);
}else{
    ris = ris + s1 + s2;;
    remainder = 0;
    return auxSum (s1, s2, ris, remainder);
}

int remainder = 0;
auxSum(s1, s2, ris, remainder);

return;