char* kangaroo(int x1, int v1, int x2, int v2) {
    
    char *s = malloc (10 * sizeof(char));
    int relativeSpeed;
    
    //if speed of kangaroo 1 is slower or equal to kangaroo 2,    they will never meet
    if (v1 <= v2) {
        s = "NO";
    }
    else {
        //finding relative speed
        relativeSpeed = x2 - x1;
        
        //If relativeSpeed % (v1 - v2) == 0, then the kangaroos will meet at some future time.
        if (relativeSpeed % (v1 - v2) == 0) {
            s = "YES";
        }
        else {
            s = "NO";
        }
    }
    
    return s;
    free(s);
}