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 Go (1.6) (judge verdict: Wrong Answer)

package main
import (
  "fmt"
  "math"
)
func main() {
  var x,n,y,z int64
  fmt.Scan(&x)
  n = int64(math.Floor(math.Sqrt(float64(1+8*x))))
  n = (n-1)/2
  if (n*(n+1))/2 == x {
    fmt.Println(n)
  } else {
    y = x - (n*(n+1))/2
    z = ((n+1)*(n+2))/2 - x
    fmt.Println(min(n+y,n+2+z))
  }
}
func min(n int64,m int64) int64 {
  if n < m {
    return n
  } else {
    return m
  }
}