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

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */

//package codeforces;

import java.util.*;

public class bet {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        int [] a=new int[5];
        for(int i=0;i<5;i++)
        a[i]=sc.nextInt();
        int s=1;
        try{
        for(int i=0;i<5;i++)
        {
            if(a[i]==a[i+1])
                s++;
        }
        }catch(Exception e)
        {
            System.out.print("");
        }
        if(s==5)
            System.out.println("-1");
        else
        {
        Arrays.sort(a);
        int min=a[0],max=a[4];
        while(min!=max && max-min>=2)
        {
            a[0]=a[0]+1;
            a[4]=a[4]-1;
            Arrays.sort(a);
            min=a[0];
            max=a[4];
        }
        int count=1;
        try{
        for(int i=0;i<5;i++)
        {
            if(a[i]==a[i+1])
                count++;
            else
                break;
        }
        }catch(Exception e)
        {
            System.out.print("");
        }
        if(count==5)
            System.out.println(a[0]);
        else
            System.out.println("-1");
    }
    
}
}