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# (.NET Core 3.1.201) (judge verdict: Wrong Answer)

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

namespace ABC
{
    class ProgramC
    {
        static void Main(string[] args)
        {
            var x = long.Parse(Console.ReadLine());
            var list = new List<long>();
            list.Add(1);
            var result = 0;
            var y = 0L;
            var isOver = false;

            for (var i = 2; i < x; i++)
            {
                var a = list.Last() + i;
                if ((a > x / 2) && !isOver)
                {
                    y = a;
                    isOver = true;
                }
                else if (a > x)
                {
                    Console.WriteLine(x - y);
                    return;
                }
                else if (a == x)
                {
                    Console.WriteLine(i);
                    return;
                }

                list.Add(a);
            }

            Console.WriteLine(result);
        }
    }
}