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.lang.*;
import java.io.*;

public class Main {

    public static void main(String[] args) {
        InputStream inputstream = System.in;
        OutputStream outputstream = System.out;
        InputReader in = new InputReader(inputstream);
        PrintWriter out = new PrintWriter(outputstream);
 //       int T;
        //      T = Integer.parseInt(in.readString());
        //      for (int i = 1; i <= T; i++) {
        Task solver = new Task();
        solver.solve(1, in, out);
        //      } 
        out.close();
    }
}

class Task {

    public void solve(int testno, InputReader in, PrintWriter out) {
        int c1=in.readInt();
        int c2=in.readInt();
        int c3=in.readInt();
        int c4=in.readInt();
        int c5=in.readInt();
        
        int sum=c1+c2+c3+c4+c5;
        if(sum%5==0){
           out.println(sum/5); 
        }else{
            out.println(-1);
        }
    }

}

class InputReader {

    private InputStream stream;
    private byte[] buf = new byte[1024];
    private int curChar;
    private int numChars;

    public InputReader(InputStream stream) {
        this.stream = stream;
    }

    public int read() {
        if (numChars == -1) {
            throw new InputMismatchException();
        }
        if (curChar >= numChars) {
            curChar = 0;
            try {
                numChars = stream.read(buf);
            } catch (IOException e) {
                throw new InputMismatchException();
            }
            if (numChars <= 0) {
                return -1;
            }
        }
        return buf[curChar++];
    }

    public int readInt() {
        int c = read();
        while (isSpaceChar(c)) {
            c = read();
        }
        int sgn = 1;
        if (c == '-') {
            sgn = -1;
            c = read();
        }
        int res = 0;
        do {
            if (c < '0' || c > '9') {
                throw new InputMismatchException();
            }
            res *= 10;
            res += c - '0';
            c = read();
        } while (!isSpaceChar(c));
        return res * sgn;
    }

    public String readString() {
        int c = read();
        while (isSpaceChar(c)) {
            c = read();
        }
        StringBuffer res = new StringBuffer();
        do {
            res.appendCodePoint(c);
            c = read();
        } while (!isSpaceChar(c));
        return res.toString();
    }

    public long readLong() {
        return Long.parseLong(readString());
    }

    public double readDouble() {
        return Double.parseDouble(readString());
    }

    public float readFloat() {
        return Float.parseFloat(readString());
    }

    public static boolean isSpaceChar(int c) {
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }
}
