Problem:
Score : 200 points

Problem Statement

There is a kangaroo at coordinate 0 on an infinite number line that runs from left to right, at time 0.
During the period between time i-1 and time i, the kangaroo can either stay at his position, or perform a jump of length exactly i to the left or to the right.
That is, if his coordinate at time i-1 is x, he can be at coordinate x-i, x or x+i at time i.
The kangaroo's nest is at coordinate X, and he wants to travel to coordinate X as fast as possible.
Find the earliest possible time to reach coordinate X.

Constraints

X is an integer.

1≤X≤10^9

Input

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

X

Output

Print the earliest possible time for the kangaroo to reach coordinate X.

Sample Input 1

6

Sample Output 1

3

The kangaroo can reach his nest at time 3 by jumping to the right three times, which is the earliest possible time.

Sample Input 2

2

Sample Output 2

2

He can reach his nest at time 2 by staying at his position during the first second, and jumping to the right at the next second.

Sample Input 3

11

Sample Output 3

5

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static string InputPattern = "InputX";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("6");
            //3
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("2");
            //2
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("11");
            //5
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static void Main()
    {
        List<string> InputList = GetInputList();
        int X = int.Parse(InputList[0]);

        int SumVal = 0;
        for (int I = 1; I <= int.MaxValue; I++) {
            SumVal += I;
            if (Math.Abs(X) <= SumVal) {
                Console.WriteLine(I);
                break;
            }
        }
    }

    static int SolveNaive(int pX)
    {
        //到達可能な座標のSet
        var VisitedSet = new HashSet<int>();
        VisitedSet.Add(0);

        int I = 0;
        while (true) {
            if (VisitedSet.Contains(pX)) {
                return I;
            }
            I++;

            foreach (int EachInt in VisitedSet.ToArray()) {
                VisitedSet.Add(EachInt + I);
                VisitedSet.Add(EachInt - I);
            }
        }

    }
}