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.BufferedReader;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class Main{
    public static void main(String[]args){
        fastSC sc = new fastSC();
        int sum = 0;
        int n = 5;
        while(n--!=0){
            sum+=sc.nextInt();
        }
        System.out.println(sum%5==0 && sum!=0?sum/5:-1);
    }
    static class fastSC{
        BufferedReader br;
        StringTokenizer st;
        private fastSC(){
            br = new BufferedReader(new InputStreamReader(System.in));
        }
        String next(){
            while(st==null || !st.hasMoreElements()){
                try{
                    st = new StringTokenizer(br.readLine());
                }catch(Exception e){
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }
        int nextInt(){
            return Integer.parseInt(next());
        }
   }
}