题目：
A. Vanya and Cubes
time limit per test
1 second
memory limit per test
256 megabytes
input
standard input
output
standard output
Vanya got n cubes. He decided to build a pyramid from them. Vanya wants to build the pyramid as follows: the top level of the pyramid must consist of 1 cube, the second level must consist of 1?+?2?=?3 cubes, the third level must have 1?+?2?+?3?=?6 cubes, and so on. Thus, the i-th level of the pyramid must have 1?+?2?+?...?+?(i?-?1)?+?i cubes.
Vanya wants to know what is the maximum height of the pyramid that he can make using the given cubes.
Input
The first line contains integer n (1?≤?n?≤?104) ? the number of cubes given to Vanya.
Output
Print the maximum possible height of the pyramid in the single line.
Sample test(s)
input
output
input
25
output
Note
Illustration to the second sample:
没啥好说的。模拟一下就可以了，还wa一发不应该..
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