Problem:
Score : 300 points

Problem Statement

Dolphin resides in two-dimensional Cartesian plane, with the positive x-axis pointing right and the positive y-axis pointing up.

Currently, he is located at the point (sx,sy). In each second, he can move up, down, left or right by a distance of 1.

Here, both the x- and y-coordinates before and after each movement must be integers.

He will first visit the point (tx,ty) where sx < tx and sy < ty, then go back to the point (sx,sy), then visit the point (tx,ty) again, and lastly go back to the point (sx,sy).

Here, during the whole travel, he is not allowed to pass through the same point more than once, except the points (sx,sy) and (tx,ty).

Under this condition, find a shortest path for him.

Constraints

-1000 ≤ sx < tx ≤ 1000

-1000 ≤ sy < ty ≤ 1000

sx,sy,tx and ty are integers.

Input

The input is given from Standard Input in the following format:

sx sy tx ty

Output

Print a string S that represents a shortest path for Dolphin.

The i-th character in S should correspond to his i-th movement.

The directions of the movements should be indicated by the following characters:

U: Up

D: Down

L: Left

R: Right

If there exist multiple shortest paths under the condition, print any of them.

Sample Input 1

0 0 1 2

Sample Output 1

UURDDLLUUURRDRDDDLLU

One possible shortest path is:

Going from (sx,sy) to (tx,ty) for the first time: (0,0) → (0,1) → (0,2) → (1,2)

Going from (tx,ty) to (sx,sy) for the first time: (1,2) → (1,1) → (1,0) → (0,0)

Going from (sx,sy) to (tx,ty) for the second time: (0,0) → (-1,0) → (-1,1) → (-1,2) → (-1,3) → (0,3) → (1,3) → (1,2)

Going from (tx,ty) to (sx,sy) for the second time: (1,2) → (2,2) → (2,1) → (2,0) → (2,-1) → (1,-1) → (0,-1) → (0,0)

Sample Input 2

-2 -2 1 1

Sample Output 2

UURRURRDDDLLDLLULUUURRURRDDDLLDL

Solution in Kotlin (1.0.0) (judge verdict: Wrong Answer)

import java.io.PrintWriter
import java.util.*
import java.math.*
import java.lang.*
import kotlin.comparisons.*

val pw = PrintWriter(System.out)
val MOD: Long = (1e+9 + 7).toLong()

fun main(args: Array<String>) {
    val (sx, sy, tx, ty) = readListOfInt()
    var going = ""
    for(i in sx until tx) {
        going += "R"
    }
    for(i in sy until ty) {
        going += "U"
    }
    var returnRoute = ""
    for(c in going) {
        when(c.toString()) {
            "R" -> returnRoute += "L"
            else -> returnRoute += "D"
        }
    }

    println(going + returnRoute)
    pw.flush()
}

/****** Declared Functions and Data Structures ******/
// IO
fun read() = readLine()!!
fun readInt() = read().toInt()
fun readLong() = read().toLong()
fun readDouble() = read().toDouble()
fun readListOfString() = read().split(" ")
fun readListOfInt() = readListOfString().map { it.toInt() }
fun readListOfLong() = readListOfString().map { it.toLong() }
fun readListOfDouble() = readListOfString().map { it.toDouble() }

fun Double.format(digits: Int) = String.format("%.${digits}f", this)
fun Float.format(digits: Int) = String.format("%.${digits}f", this)

fun print(value: Any) { pw.print(value) }
fun println(value : Any) { pw.println(value) }

// Extensions
fun<T> List<T>.toBuckets(): Map<T, Int>
    = this.groupBy { it }.mapValues { it.value.size }

fun<T: Comparable<T>> List<T>.upperBound(element: T, fromIndex: Int=0, toIndex: Int=this.size): Int {
    var l = fromIndex
    var r = toIndex
    while(l < r) {
        val mid = (l+r) / 2
        if(element >= this[mid]) { 
            l = mid + 1
        } else {
            r = mid
        }
    }
    return l
}

fun<T: Comparable<T>> List<T>.lowerBound(element: T, fromIndex: Int=0, toIndex: Int=this.size): Int {
    var l = fromIndex
    var r = toIndex
    while(l < r) {
        val mid = (l+r) / 2
        if(element <= this[mid]) { 
            r = mid
        } else {
            l = mid + 1
        }
    }
    return r 
}

fun <K, V> Map<K, V>.toMutableMap(): MutableMap<K, V> = HashMap(this)

// Util Functions
fun Long.makeDivisors(): List<Long> {
    val divisors = mutableListOf<Long>()
    for(i in 1L .. Math.sqrt(this.toDouble()).toLong()+1L) {
        if(this % i == 0L) {
            divisors.add(i)
            if(i != this/i) {
                divisors.add(this/i)
            }
        }
    }
    return divisors.distinct().sorted()
}

fun Long.isPrime(): Boolean {
    if(this==1L) return false
    for(i in 2L .. Math.sqrt(this.toDouble()).toLong() + 1L) {
        if(this % i == 0L && this != i) {
            return false
        }
    }
    return true
}


fun<T> permutations(src: List<T>): List<List<T>> {
    if(src.size == 1) return listOf(src)
    val perms = mutableListOf<List<T>>()
    val insertElement = src[0]
    permutations(src.drop(1)).forEach { perm ->
        for(i in 0..perm.size) {
            val newPerm = perm.toMutableList()
            newPerm.add(i, insertElement)
            perms.add(newPerm.toList())
        }
    }
    return perms
}

// return nCr, if you want nCr, please access v[n][r]
fun combinations(n: Long): List<List<Long>> {
    val v = (0..n).map { (0..n).map{0L}.toMutableList() }.toMutableList()
    for(i in 0 until v.size) {
        v[i][0] = 1L
        v[i][i] = 1L
    }
    for(i in 0 until v.size) {
        for(j in 1 until i) {
            v[i][j] = (v[i-1][j-1] + v[i-1][j]) % MOD
        }
    }
    return v.map { it.toList() }.toList()
}

fun repeatedModPow(a: Long, n: Long, mod: Long): Long = 
    when {
        n == 1L -> a % mod
        n % 2L == 1L -> (a * repeatedModPow(a, n-1L, mod)) % mod
        else -> {
            val tmp = repeatedModPow(a, n / 2L, mod)
            (tmp * tmp) % mod
        }
    }

// return nCr % MOD
fun modnCkWithoutArray(n: Long, k: Long, mod: Long): Long {
    var ret = 1L
    val min = Math.min(k, n-k)
    // まず分母の計算
    for(i in n downTo (n - min + 1)) {
        ret = ret * i % mod
    }
    for(i in min downTo 1) {
        ret = ret * repeatedModPow(i, (mod - 2L), mod) % mod
    }
    return ret
}

// return nCr % MOD
fun modnCk(n: Int, k: Int, mod: Long): Long {
    val factorials = Array(n+1) { 0L }.apply {
        this[0] = 1L
        if(this.size > 1) this[1] = 1L
    }
    val inverses = Array(n+1) { 0L }.apply {
        this[0] = 1L
        if(this.size > 1) this[1] = 1L
    }
    val factorialInverses = Array(n+1) { 0L }.apply {
        this[0] = 1L
        if(this.size > 1) this[1] = 1L
    }
    for(i in 2 .. n) {
        factorials[i] = (factorials[i-1] * i.toLong() % mod)
        inverses[i] = (mod - inverses[mod.toInt() % i] * (mod / i.toLong()) % mod)
        factorialInverses[i] = (factorialInverses[i-1] * inverses[i] % mod)
    }

    return factorials[n] % mod * (factorialInverses[k] * factorialInverses[n-k] % mod) % mod
}

// should be a >= b
fun gcd(a: Long, b: Long): Long = 
    if(b == 0L) a else gcd(b, a % b)

// shoud be a >= b
fun lcm(a: Long, b: Long): Long = 
    a / gcd(a, b) * b

fun sumDigits(num_: Long): Long {
    var num = num_
    var rtn: Long = 0
    while(num != 0.toLong()) {
        val tmp = num % 10
        rtn += tmp
        num /= 10
    }
    return rtn
}

fun Double.isDecimal(): Boolean = ((this - Math.floor(this)) != 0.0)

// Data Structures

class UnionFind(private val n: Int) {
    private val parent: Array<Int> = Array(n) { 0 }
    private val rank: Array<Int> = Array(n) { 0 }
    private val size: Array<Int> = Array(n) { 0 }

    init {
        for(i in 0 until n) {
            parent[i] = i
            rank[i] = 1
            size[i] = 1
        }
    }

    fun union(u: Int, v: Int) {
        var x = root(u)
        var y = root(v)
        if(x != y) {
            if(rank[y] < rank[x]) {
                val tmp = y 
                y = x
                x = tmp 
            }
            parent[x] = y
            rank[y] += rank[x]
            rank[x] = -1
            val totalSize = size[x] + size[y]
            size[x] = totalSize
            size[y] = totalSize
        }
    }

    fun root(u: Int): Int {
        var x = u
        while(parent[x] != x) {
            x = parent[x]
        }
        return x 
    }

    fun size(u: Int): Int = size[root(u)]
}