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.util.*;
public class HelloWorld{

     public static void main(String []args){
        Scanner sc = new Scanner(System.in);
        int a,b,c,d,e;
        double s=0;
        a=sc.nextInt();
        b=sc.nextInt();
        c=sc.nextInt();
        d=sc.nextInt();
        e=sc.nextInt();
        s=a+b+c+d+e;
        if (a>=0 && a<=100 && b>=0 && b<=100 && c>=0 && c<=100 && d>=0 && d<=100 && e>=0 && e<=100){
        if(s%5==0){System.out.print((int)(s/5));} else {System.out.print("-1");}}
        if(s==0){System.out.print("-1");}
     }
}