CF478A.Initial Bet

There are five people playing a game called "Generosity". Each person gives some non-zero number of coins b as an initial bet. After all players make their bets of b coins, the following operation is repeated for several times: a coin is passed from one player to some other player.

Your task is to write a program that can, given the number of coins each player has at the end of the game, determine the size b of the initial bet or find out that such outcome of the game cannot be obtained for any positive number of coins b in the initial bet.

有五个人在玩一个名为“慷慨”的游戏。每位玩家初始下注一些非零数量的硬币 $b$。在所有玩家都下注了 $b$ 枚硬币后，重复执行以下操作若干次：一枚硬币从一名玩家传递给另一名玩家。

你的任务是编写一个程序，根据游戏结束时每位玩家所拥有的硬币数量，确定初始下注量 $b$ 的大小；或者判断：对于任意正整数 $b$，该最终局面都不可能出现。

The input consists of a single line containing five integers _c_1, _c_2, _c_3, _c_4 and _c_5 — the number of coins that the first, second, third, fourth and fifth players respectively have at the end of the game (0 ≤ _c_1, _c_2, _c_3, _c_4, _c_5 ≤ 100).

输入包含一行，其中包含五个整数 $c_1,\,c_2,\,c_3,\,c_4$ 和 $c_5$ —— 分别表示游戏结束时第一、第二、第三、第四和第五位玩家所拥有的硬币数量（$0\leq c_1,\,c_2,\,c_3,\,c_4,\,c_5\leq 100$）。

Print the only line containing a single positive integer b — the number of coins in the initial bet of each player. If there is no such value of b, then print the only value "-1" (quotes for clarity).

输出唯一一行，包含一个正整数 $b$ —— 即每位玩家初始下注的硬币数量。若不存在满足条件的 $b$ 值，则输出唯一的值 -1（引号仅用于明确显示）。

In the first sample the following sequence of operations is possible:

1. One coin is passed from the fourth player to the second player;
2. One coin is passed from the fourth player to the fifth player;
3. One coin is passed from the first player to the third player;
4. One coin is passed from the fourth player to the second player.

在第一个样例中，可以执行以下操作序列：

1. 第四名玩家向第二名玩家传递一枚硬币；
2. 第四名玩家向第五名玩家传递一枚硬币；
3. 第一名玩家向第三名玩家传递一枚硬币；
4. 第四名玩家向第二名玩家传递一枚硬币。