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 Main {

   
    public static void main(String[] args) {
      Scanner sc=new Scanner(System.in);
      int c1,c2,c3,c4,c5;
      c1=sc.nextInt();
       c2=sc.nextInt();
        c3=sc.nextInt();
         c4=sc.nextInt();
          c5=sc.nextInt();
          int sum=0;
                  sum+=c1+c2+c3+c4+c5;
          
           if(sum%5==1){
              System.out.print(-1);
          }
          else{
              int ans=sum/5;
              if(ans!=0)
              System.out.print(ans);
              else{
                  System.out.print(-1);
              }
          }
                  
      
    }
    
}
