func kangaroo(x1: Int, v1: Int, x2: Int, v2: Int) -> String {
    var currentLocation1 = x1
    var currentLocation2 = x2
    guard v2 < v1 else { return "NO" }

    while(differenceInLocations >= 0) {
        if differenceInLocations == 0 {
            return "YES"
        }
        currentLocation1 = currentLocation1 + v1
        currentLocation2 = currentLocation2 + v2
        if differenceInLocations > currentLocation2 - currentLocation1 {
            differenceInLocations = currentLocation2 - currentLocation1
    }

    return "NO"
}