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.OutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.InputMismatchException;
import java.util.StringTokenizer;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
	public static void main(String[] args) {
		InputStream inputStream = System.in;
		OutputStream outputStream = System.out;
		QuickScanner in = new QuickScanner(inputStream);
		PrintWriter out = new PrintWriter(outputStream);
		TaskA solver = new TaskA();
		solver.solve(1, in, out);
		out.close();
	}
}

class TaskA {
    public void solve(int testNumber, QuickScanner in, PrintWriter out) {
        int v[]=in.nextInts(5);
        int sum=0;
        for(int i:v)
            sum+=i;
        if(sum%5==0)
            out.println(sum/5);
        else
            out.println(-1);
    }
}

class QuickScanner {
    BufferedReader in;
    StringTokenizer token;
    String delim;
    public QuickScanner(InputStream inputStream)
    {
        this.in=new BufferedReader(new InputStreamReader(inputStream));
        this.delim=" \n\t";
        this.token=new StringTokenizer("",delim);
    }

    public boolean hasNext()
    {
        while(!token.hasMoreTokens())
        {
            try {
                String s=in.readLine();
                if(s==null) return false;
                token=new StringTokenizer(s,delim);
            } catch (IOException e) {
                throw new InputMismatchException();
            }
        }
        return true;
    }

    public String next()
    {
        hasNext();
        return token.nextToken();
    }

    public int nextInt()
    {
        return Integer.parseInt(next());
    }

    public int[] nextInts(int n)
    {
        int[] res=new int[n];
        for(int i=0;i<n;i++)
            res[i]=nextInt();
        return res;
    }
}

