char* concat(const char *s1, const char *s2){
    const size_t len1 = strlen(s1);
    const size_t len2 = strlen(s2);
    char *result = malloc(len1 + len2 + 1);
    memcpy(result, s1, len1);
    memcpy(result + len1, s2, len2 + 1);
    return result;
}