Codeforces Round #273 (Div. 2)-A. Initial Bet

http://codeforces.com/contest/478/problem/A

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.

Input

The input consists of a single line containing five integers c1, c2, c3, c4 and c5 — the number of coins that the first, second, third, fourth and fifth players respectively have at the end of the game (0 ≤ c1, c2, c3, c4, c5 ≤ 100).

Output

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).

Sample test(s)
input
2 5 4 0 4
output
3
input
4 5 9 2 1
output
-1

Note

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

One coin is passed from the fourth player to the second player;
One coin is passed from the fourth player to the fifth player;
One coin is passed from the first player to the third player;
One coin is passed from the fourth player to the second player.

解题思路：能够将硬币平均分给5个人，有一个trick就是开始都为0

1 #include <stdio.h>
2
3 int main(){
4     int a, sum = 0, t = 5;
5     while(t--){
6         scanf("%d", &a);
7         sum += a;
8     }
9     printf("%d ", sum >= 5 && sum % 5 == 0 ? sum / 5 : -1);
10     return 0;
11 }