题意：有n个灯笼。第一个的高度是A，最后一个是B。灯笼的关系给出。并要求每一个灯笼的高度是非负数的。求最低的B

思路：推出公式：H[i]=2*H[i-1]+2-H[i-2],然后枚举H[2],在知道H[1]的情况下就能求出全部的高度，然后推断是否都是非负数

#include <iostream>

#include <cstring>

#include <algorithm>

#include <cstdio>

using namespace std;

const int MAXN = 1500;

int n;

double A, B, H[MAXN];

int check(double cnt) {

	H[1] = cnt;

	for (int i = 2; i < n; i++) {

		H[i] = 2*H[i-1] + 2 - H[i-2];

		if (H[i] < 0)

			return 0;

	}

	B = H[n-1];

	return true;

}

int main() {

	while (scanf("%d%lf", &n, &A) != EOF) {

		H[0] = A;

		double l = -1, r = MAXN;

		while (r-l > 1e-6) {

			double mid = (l+r)/2;

			if (check(mid))

				r = mid;

			else l = mid;

		}

		printf("%.2lf\n", B);

	}

	return 0;

}