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.*;
    import java.io.*;
     
    public class solution {
        public static void main(String[] args) throws IOException {
            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
            StringTokenizer st = new StringTokenizer(br.readLine());
            int c1 = Integer.parseInt(st.nextToken());
            int c2 = Integer.parseInt(st.nextToken());
            int c3 = Integer.parseInt(st.nextToken());
            int c4 = Integer.parseInt(st.nextToken());
            int c5 = Integer.parseInt(st.nextToken());
            long sum = c1 + c2 + c3 + c4 + c5;
            if (sum != 0 && sum % 5 == 0) {
                System.out.println(sum / 5);
            } else
                System.out.println(-1);
        }
     
        static class sort implements Comparator<ArrayList<Integer>> {
     
            @Override
            public int compare(ArrayList<Integer> o1, ArrayList<Integer> o2) {
                int c = o1.get(0).compareTo(o2.get(0));
                return c;
            }
     
        }
    }