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 static java.lang.System.*;
import static java.lang.Math.*;
import java.io.*;
import java.util.*;
import java.math.*;

public class Alyona {
    public static void main(String[] args) throws IOException{
        Scanner input=new Scanner(System.in);
        int sum=0;
        int sum2=0;
        for(int i=0; i<5; i++){
            int n=input.nextInt();
            sum+=n;
            sum2+=n;
            sum%=5;
        }
        if(sum!=0){
            out.println(-1);
        }
        else{
            out.println(sum2/5);
        }
    }
}