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;

namespace Although_I_Always_Use_C_plus_plus
{
	class MainClass
	{
		public static void Main(string[] args)
		{
            int x = readInt(), i;
            for (i = 1; i * (i + 1) / 2 < x; i++) {}
            Console.Write(i);
		}

		private static int readInt()
		{
			char c;
			while ((c = (char)Console.Read()) > 0)
				if (c >= '0' && c <= '9' || c == '-') break;
			int res = 0, multi = (c == '-' ? -1 : 1);
			do
			{
				if (c == '-')
					continue;
				else if (c >= '0' && c <= '9')
					res = res * 10 + c - '0';
				else
					break;
			} while ((c = (char)Console.Read()) > 0);
			return res;
		}
	}
}