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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <cstdio>
#include <vector>
#include <algorithm>
#include <climits>

using namespace std;

typedef long long ll;

void ignore() {
    while (getchar() != '\n');
}

int main()
{
    //freopen("in.txt", "r", stdin);
    //freopen("out.txt", "w", stdout);
    //freopen("debug.txt", "w", stderr);

    ll X;

    scanf("%lld", &X);

    ll prevX = X / 2;
    ll ans = 0;

    while (1) {
        X /= 2;
        ans += prevX - X;

        if (X <= 1) break;

        prevX = X;

        if (X % 2 == 1) X++;
    }

    if (ans == 0) ans++;
    if (X == 1) ans++;

    printf("%lld\n", ans);

    return 0;
}