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: Wrong Answer)

using System;
using System.Globalization;
using System.IO;
using System.Linq;

namespace AtCoder
{
    internal class Program
    {
        static void Main(string[] args)
        {
            var sr = new InputReader(Console.In);
            //var sr = new InputReader(new StreamReader("input.txt"));
            using (var sw = Console.Out)
            //using (var sw = new StreamWriter("output.txt"))
            {
                Solve(sr, sw);
            }
        }


        internal static void Solve(InputReader sr, TextWriter sw)
        {
            var n = sr.NextInt32();

            var i = 0;
            var sum = 0;
            while (true)
            {
                if (sum == n)
                {
                    sw.WriteLine(i);
                    return;
                }
                sum += ++i;
            }
        }
    }
    internal class InputReader : IDisposable
    {
        private bool isDispose;
        private readonly TextReader sr;

        public InputReader(TextReader stream)
        {
            sr = stream;
        }

        public void Dispose()
        {
            //Dispose(true);
            GC.SuppressFinalize(this);
        }

        public string NextString()
        {
            var result = sr.ReadLine();
            return result;
        }

        public int NextInt32()
        {
            return int.Parse(NextString());
        }

        public long NextInt64()
        {
            return long.Parse(NextString());
        }

        public string[] NextSplitStrings()
        {
            return NextString()
                .Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);
        }

        public string[] NextSplitInts()
        {
            return NextString()
                .Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);
        }

        public T[] ReadArray<T>(Func<string, CultureInfo, T> func)
        {
            return NextSplitStrings()
                .Select(s => func(s, CultureInfo.InvariantCulture))
                .ToArray();
        }

        public T[] ReadArrayFromString<T>(Func<string, CultureInfo, T> func, string str)
        {
            return str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries)
                .Select(s => func(s, CultureInfo.InvariantCulture))
                .ToArray();
        }

        //protected void Dispose(bool dispose)
        //{
        //    if (isDispose) return;
        //    if (dispose)
        //    {
        //        sr.Close();
        //    }
        //    isDispose = true;
        //}

        //~InputReader()
        //{
        //    Dispose(false);
        //}
    }
}