99 100 100 100 100
 57 83 11 4 93
 99 99 99 99 99
 100 0 0 0 0
 0 1 2 3 4
 93 100 99 90 98
 87 38 19 33 100
 1 1 1 1 1
 0 0 0 0 1
 2 3 4 5 6
 1 2 1 2 3
 0 0 0 0 0
 100 100 100 100 100
 43 83 1 0 23
 43 83 1 100 23
 56 0 0 0 4
 99 98 98 99 100

-1
 -1
 99
 20
 2
 96
 -1
 1
 -1
 4
 -1
 -1
 100
 30
 50
 12
 -1

import java.io.*;
import java.util.*;
import java.math.BigInteger;

public class Test{
 
public static void main(String[] args) throws IOException{
    BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
    String[] s=br.readLine().split(" ");
    int n=0;
    for(int i=0;i<5;i++){
    	n+=Integer.parseInt(s[i]);
   }
   if(n==0){
	System.out.println(0);
   }
   else if(n%5==0){
	System.out.println(n/5);
   }
   else{
	System.out.println(-1);
	}
}
}