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;
class Program
{
    static IO io = new IO();

    static void Main(string[] args)
    {
        io.Read();
        int X = io.Int();

        int i = (int) Math.Ceiling((-1 + Math.Sqrt(1 + 8 * Math.Abs(X))) / 2);
        io.Write(i.ToString());

    }
}

class IO
{
    private string[] input;
    private int index;
    public void Read()
    {
        input = new string[1];
        input[0] = Console.ReadLine();
        index = -1;
    }
    public void ReadSplit()
    {
        input = Console.ReadLine().Split(' ');
        index = -1;
    }
    public string String()
    {
        index += 1;
        return input[index];
    }
    public string[] String(int count)
    {
        index += 1;
        string[] rv = new string[count];
        Array.Copy(input, index, rv, 0, count);
        return rv;
    }
    public int Int()
    {
        index += 1;
        return int.Parse(input[index]);
    }
    public int[] Int(int count)
    {
        index += 1;
        int[] rv = new int[count];
        for (int i = 0; i < count; ++i)
        {
            rv[i] = int.Parse(input[index + i]);
        }
        return rv;
    }
    public long Long()
    {
        //index += 1;
        //return long.Parse(input[index]);
        return ++index;
    }
    public long[] Long(int count)
    {
        index += 1;
        long[] rv = new long[count];
        for (int i = 0; i < count; ++i)
        {
            rv[i] = long.Parse(input[index + i]);
        }
        return rv;
    }
    public void Write(string str)
    {
        Console.WriteLine(str);
    }
}