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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.Collections;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        
        //
		int X = Integer.parseInt(br.readLine());
		int t = 0;
		int sum = 0;
		
		
		List<Integer> l = new ArrayList<Integer>(Arrays.asList(0));
		List<Integer> l2 = new ArrayList<Integer>();
		
		while (true) {
			sum += ++t;
        	//System.out.println("t=["+t+"], sum=["+sum+"]");
        	//if (t > 10) { return; }
        	if (sum >= X) { System.out.println(t); return; }
		}


        //System.out.println(t);
        //return;
    }
    //-----------------------------------------------------------------
    // Debug.Print
    static void dp(String s) {
		System.out.println(s);
	}
    static void dp(StringBuilder s) { dp(s.toString()); }
    static void dp(int i) { dp(String.valueOf(i)); }
    static void dp(long i) { dp(String.valueOf(i)); }


	//-----------------------------------------------------------------
	// 入力値確認：リスト
	static void checkInput(List l) { System.out.println(l); }
	//-----------------------------------------------------------------
	// 入力値確認：配列、二次元、int型
	static void checkInput(int[][] i2) {
		StringBuilder sb = new StringBuilder();
		for(int[] i1: i2){
			for(int i: i1) sb.append("[" + i + "]");
			sb.append(System.getProperty("line.separator"));
		}
		System.out.println(sb);
	}

}