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 Python (3.4.3) (judge verdict: Accepted)

def main():
    x = int(input())

    t = 1
    while t * (t + 1) < x * 2:
        t += 1
    print(t)

    # [1,t]総和がXを超えるとき
    # 超過分に相当する距離のジャンプが区間に含まれる


if __name__ == '__main__':
    main()

# import sys
# input = sys.stdin.readline
# 
# sys.setrecursionlimit(10 ** 7)
# 
# (int(x)-1 for x in input().split())
# rstrip()
#
# def binary_search(*, ok, ng, func):
#     while abs(ok - ng) > 1:
#         mid = (ok + ng) // 2
#         if func(mid):
#             ok = mid
#         else:
#             ng = mid
#     return ok