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.*;
import java.util.*;
public class Main {

    public static void main(String[] args) 
    {	
        InputStream inputstream = System.in;
        OutputStream outputstream = System.out;
        InputReader in = new InputReader(inputstream);
        OutputWriter out = new OutputWriter(outputstream);
        mysolver mysol = new mysolver();
        mysol.solve(in, out);
        out.close();
    }

}

class mysolver {
    public void solve(InputReader in,OutputWriter out)
    {
    	PrintWriter pout = new PrintWriter(new BufferedOutputStream(System.out));
    	int A[] = new int[5];
    	int sum = 0;
    	for(int i=0;i<5;i++)
    	{
    		A[i] = in.readInt();
    		sum = sum + A[i];
    	}
    	if(sum %5==0)
    	{
    		if(sum == 0)
    			pout.println(-1);
    		else
    			pout.println((int)(sum/5));
    	}
    	else
    	{
    		pout.println(-1);
    	}
    	pout.close();
    }

}
class InputReader {
 
    private InputStream stream;
    private byte[] buf = new byte[1024];
    private int curChar;
    private int numChars;
    private SpaceCharFilter filter;
 
    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();
        StringBuilder res = new StringBuilder();
        do {
            res.appendCodePoint(c);
            c = read();
        } while (!isSpaceChar(c));
        return res.toString();
    }
 
    public boolean isSpaceChar(int c) {
        if (filter != null)
            return filter.isSpaceChar(c);
        return isWhitespace(c);
    }
 
    public static boolean isWhitespace(int c) {
        return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }
 
    public String next() {
        return readString();
    }
 
    public interface SpaceCharFilter {
        public boolean isSpaceChar(int ch);
    }
}
 
class OutputWriter {
    private final PrintWriter writer;
 
    public OutputWriter(OutputStream outputStream) {
        writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
    }
 
    public OutputWriter(Writer writer) {
        this.writer = new PrintWriter(writer);
    }
 
    public void close() {
        writer.close();
    }
 
    public void printLine(int i) {
        writer.println(i);
    }
}
 
class IOUtils {
 
    public static void readIntArrays(InputReader in, int[]... arrays) {
        for (int i = 0; i < arrays[0].length; i++) {
            for (int j = 0; j < arrays.length; j++)
                arrays[j][i] = in.readInt();
        }
    }
 
    }