Problem:
Score : 600 points

Problem Statement

Given is an integer N.
Find the minimum possible positive integer k such that (1+2+\cdots+k) is a multiple of N.
It can be proved that such a positive integer k always exists.

Constraints

1 \leq N \leq 10^{15}

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N

Output

Print the answer in a line.

Sample Input 1

11

Sample Output 1

10

1+2+\cdots+10=55 holds and 55 is indeed a multple of N=11.
There are no positive integers k \leq 9 that satisfy the condition, so the answer is k = 10.

Sample Input 2

20200920

Sample Output 2

1100144

Solution in Kotlin (1.3.71) (judge verdict: Wrong Answer)

@file:Suppress("NOTHING_TO_INLINE", "EXPERIMENTAL_FEATURE_WARNING", "OVERRIDE_BY_INLINE")
//@file:OptIn(ExperimentalStdlibApi::class)

import java.io.PrintWriter
import kotlin.math.*
import kotlin.random.Random
import kotlin.collections.sort as _sort
import kotlin.collections.sortDescending as _sortDescending
import kotlin.io.println as iprintln


/** @author Spheniscine */
fun main() { _writer.solve(); _writer.flush() }
fun PrintWriter.solve() {
//    val startTime = System.nanoTime()

    val numCases = 1//readInt()
    case@ for(case in 1..numCases) {
//        print("Case #$case: ")

        val n = readLong()

        var ans = Long.MAX_VALUE

        (n+n).forEachDivisor run@{ x ->
            val y = (n + n) / x
            if(y == 1L) return@run

            val a = x.modInvOrElse(y) { return@run }
            var r = a * x - 1
            if (r < y) r += y
            ans = min(ans, r)
        }

        println(ans)
    }

//    iprintln("Time: ${(System.nanoTime() - startTime) / 1000000} ms")
}

infix fun Int.divCeil(other: Int) =
    (this / other).let { if(xor(other) >= 0 && it * other != this) it+1 else it }

inline infix fun Int.divFloor(other: Int) = Math.floorDiv(this, other)

infix fun Long.divCeil(other: Long) =
    (this / other).let { if(xor(other) >= 0 && it * other != this) it+1 else it }
inline infix fun Long.divCeil(other: Int) = divCeil(other.toLong())

inline infix fun Long.divFloor(other: Long) = Math.floorDiv(this, other)

// floor and ceiling multiples, named by analogy with Excel's MROUND
infix fun Int.mCeil(m: Int) = divCeil(m) * m
infix fun Int.mFloor(m: Int) = divFloor(m) * m
infix fun Long.mCeil(m: Long) = divCeil(m) * m
infix fun Long.mFloor(m: Long) = divFloor(m) * m

inline fun Long.modInvOrElse(base: Long, default: () -> Long): Long {
    var s = 0L
    var os = 1L
    var r = base
    var or = this % base

    while(r != 0L) {
        val q = or / r
        or = r.also { r = or - q * r }
        os = s.also { s = os - q * s }
    }

    return if(or != 1L) default()
    else os.let { if(it >= 0) it else it + base }
}

inline fun Long.forEachDivisor(yield: (Long) -> Unit) {
    if(this <= 0) return
    val tail = LongList()

    for(p in 1..this) {
        val sq = p * p
        if(sq > this) break
        if(this % p == 0L) {
            yield(p)
            if(sq != this) tail.add(this / p)
        }
    }

    for(i in tail.lastIndex downTo 0) yield(tail[i])
}

class LongList(initialCapacity: Int = 12) {
    private var _arr = LongArray(initialCapacity)
    private val capacity get() = _arr.size
    var size = 0
        private set
    inline val lastIndex get() = size - 1
    inline val indices get() = 0 until size

    constructor(copyFrom: LongArray): this(copyFrom.size) { copyFrom.copyInto(_arr); size = copyFrom.size }
    constructor(copyFrom: Collection<Long>): this(copyFrom.size) { _arr = copyFrom.toLongArray(); size = copyFrom.size }

    fun contentEquals(other: LongList): Boolean {
        return this === other || size == other.size && indices.all { this[it] == other[it] }
    }

    private fun grow(minCapacity: Int = 8) {
        val newCapacity = maxOf(minCapacity, capacity + (capacity shr 1))
        _arr = _arr.copyOf(newCapacity)
    }

    fun ensureCapacity(minCapacity: Int) { if(capacity < minCapacity) grow(minCapacity) }

    operator fun get(index: Int): Long {
        require(index in 0 until size)
        return _arr[index]
    }

    operator fun set(index: Int, value: Long) {
        require(index in 0 until size)
        _arr[index] = value
    }

    fun add(value: Long) {
        if(size == capacity) grow()
        _arr[size++] = value
    }

    fun addAll(list: LongList) {
        ensureCapacity(size + list.size)
        list._arr.copyInto(_arr, size, 0, list.size)
        size += list.size
    }

    fun add(index: Int, element: Long) {
        if(size == capacity) grow()
        _arr.copyInto(_arr, index + 1, index, size)
        size++
        set(index, element)
    }

    fun clear() { size = 0 }

    fun removeAt(index: Int): Long {
        val e = get(index)
        _arr.copyInto(_arr, index, index + 1, size)
        size--
        return e
    }

    fun indexOf(e: Long): Int {
        for(i in 0 until size) if(this[i] == e) return i
        return -1
    }

    fun remove(e: Long): Boolean {
        val i = indexOf(e)
        if(i == -1) return false
        removeAt(i)
        return true
    }

    operator fun iterator() = object: LongIterator() {
        private var pos = 0
        override fun hasNext() = pos < size
        override fun nextLong() = get(pos++)
    }

    inline fun isEmpty() = size == 0
    inline fun isNotEmpty() = size != 0

    fun pop() = _arr[--size]

    fun popToSize(s: Int) {
        require(s >= 0)
        if(s < size) size = s
    }

    fun swap(i: Int, j: Int) { val t = this[i]; this[i] = this[j]; this[j] = t }
    fun reverse() {
        for(i in 0 until size / 2) swap(i, lastIndex - i)
    }

    fun shuffle(rnd: Random = random) = _shuffle(rnd, _arr::get, _arr::set, size)
    fun sort() { shuffle(); _arr._sort(0, size) }
    fun sortDescending() { sort(); reverse() }

    fun joinToString(separator: CharSequence) = if(size == 0) "" else let {
        buildString {
            append(it[0])
            for (i in 1 until size) {
                append(separator).append(it[i])
            }
        }
    }

    override fun toString() = "[" + joinToString(", ") + "]"

    fun toLongArray() = _arr.copyOf(size)
    fun toList() = List(size, ::get)

    inline fun first() = get(0)
    inline fun last() = get(lastIndex)
}

inline fun LongList(size: Int, init: (Int) -> Long) = LongList(size).apply {
    for(i in 0 until size) { add(init(i)) }
}
inline fun LongArray.toLongList() = LongList(this)
inline fun Collection<Long>.toLongList() = LongList(this)
inline fun longListOf(vararg values: Long) = LongList(values)

fun LongList.max() = (1 until size).fold(this[0]) { acc, i -> max(acc, this[i]) }
fun LongList.min() = (1 until size).fold(this[0]) { acc, i -> min(acc, this[i]) }
fun LongList.getOrNull(i: Int) = if(i in indices) get(i) else null
inline fun LongList.count(predicate: (Long) -> Boolean) = indices.count { predicate(this[it]) }
fun LongList.copyOf() = LongList(size, ::get)

/** IO */
//@JvmField val INPUT = File("input.txt").inputStream()
//@JvmField val OUTPUT = File("output.txt").outputStream()
@JvmField val INPUT = System.`in`
@JvmField val OUTPUT = System.out

const val _BUFFER_SIZE = 1 shl 16
@JvmField val _buffer = ByteArray(_BUFFER_SIZE)
@JvmField var _bufferPt = 0
@JvmField var _bytesRead = 0

tailrec fun readChar(): Char {
    if(_bufferPt == _bytesRead) {
        _bufferPt = 0
        _bytesRead = INPUT.read(_buffer, 0, _BUFFER_SIZE)
    }
    return if(_bytesRead < 0) Char.MIN_VALUE
    else {
        val c = _buffer[_bufferPt++].toChar()
        if (c == '\r') readChar()
        else c
    }
}

fun readLine(): String? {
    var c = readChar()
    return if(c == Char.MIN_VALUE) null
    else buildString {
        while(c != '\n' && c != Char.MIN_VALUE) {
            append(c)
            c = readChar()
        }
    }
}
fun readLn() = readLine()!!

fun read() = buildString {
    var c = readChar()
    while(c <= ' ') {
        if(c == Char.MIN_VALUE) return@buildString
        c = readChar()
    }
    do {
        append(c)
        c = readChar()
    } while(c > ' ')
}
fun readInt() = read().toInt()
fun readDouble() = read().toDouble()
fun readLong() = read().toLong()
fun readStrings(n: Int) = List(n) { read() }
fun readLines(n: Int) = List(n) { readLn() }
fun readInts(n: Int) = List(n) { read().toInt() }
fun readIntArray(n: Int) = IntArray(n) { read().toInt() }
fun readDoubles(n: Int) = List(n) { read().toDouble() }
fun readDoubleArray(n: Int) = DoubleArray(n) { read().toDouble() }
fun readLongs(n: Int) = List(n) { read().toLong() }
fun readLongArray(n: Int) = LongArray(n) { read().toLong() }

@JvmField val _writer = PrintWriter(OUTPUT, false)

/** shuffles and sort overrides to avoid quicksort attacks */
private inline fun <T> _shuffle(rnd: Random, get: (Int) -> T, set: (Int, T) -> Unit, size: Int) {
    // Fisher-Yates shuffle algorithm
    for (i in size - 1 downTo 1) {
        val j = rnd.nextInt(i + 1)
        val temp = get(i)
        set(i, get(j))
        set(j, temp)
    }
}

@JvmField var _random: Random? = null
val random get() = _random ?: Random(0x594E215C123 * System.nanoTime()).also { _random = it }

fun IntArray.shuffle(rnd: Random = random) = _shuffle(rnd, ::get, ::set, size)
fun IntArray.sort() { shuffle(); _sort() }
fun IntArray.sortDescending() { shuffle(); _sortDescending() }

fun LongArray.shuffle(rnd: Random = random) = _shuffle(rnd, ::get, ::set, size)
fun LongArray.sort() { shuffle(); _sort() }
fun LongArray.sortDescending() { shuffle(); _sortDescending() }

fun DoubleArray.shuffle(rnd: Random = random) = _shuffle(rnd, ::get, ::set, size)
fun DoubleArray.sort() { shuffle(); _sort() }
fun DoubleArray.sortDescending() { shuffle(); _sortDescending() }

fun CharArray.shuffle(rnd: Random = random) = _shuffle(rnd, ::get, ::set, size)
inline fun CharArray.sort() { _sort() }
inline fun CharArray.sortDescending() { _sortDescending() }

inline fun <T: Comparable<T>> Array<out T>.sort() = _sort()
inline fun <T: Comparable<T>> Array<out T>.sortDescending() = _sortDescending()
inline fun <T: Comparable<T>> MutableList<out T>.sort() = _sort()
inline fun <T: Comparable<T>> MutableList<out T>.sortDescending() = _sortDescending()

fun `please stop removing these imports IntelliJ`() { iprintln(max(1, 2)) }

/** additional commons */
inline fun <T> Iterable<T>.sumByLong(func: (T) -> Long) = fold(0L) { acc, t -> acc + func(t) }
inline fun <T> Sequence<T>.sumByLong(func: (T) -> Long) = fold(0L) { acc, t -> acc + func(t) }
inline fun <T> Array<T>.sumByLong(func: (T) -> Long) = fold(0L) { acc, t -> acc + func(t) }
fun IntArray.sumLong() = fold(0L, Long::plus)