Given a total of N regular hexagon , find out how many rows of pyramid we can build. Note that every k-th row must have exactly k hexagon.

Sample Input: 60

Sample Output: 10

Given 60 regular hexagon, we can build 10 rows of pyramid shown as below.

数据结构和算法：初级练习题# 11 – 构建金字塔

给定一组N个正六边形，计算用它们可以最多构建几层高的金字塔，每一层的正六边形数量和层数相等。

输入样例：60

输出样例：10

如上图所示，60个正六边形可以构建10层完整的金字塔，剩余五个不足构建下一层金字塔。

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