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.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

public class A {

	public static void main(String[] args) {

		InputReader in = new InputReader();
		PrintWriter out = OutputWriter();
		
		int sum = 0;
		int res = 0;
		for(int i=0; i<5; i++){
			sum += in.nextInt();
		}
		if(sum % 5 == 0 && sum>0) res = sum/5;
		else res = -1;
		out.println(res);
		out.close();
		
	}
	
	public static PrintWriter OutputWriter(){
		return new PrintWriter(System.out);
	}
	
	public static PrintWriter OutputWriter(String filename) throws IOException {
		return new PrintWriter(new BufferedWriter(new FileWriter(filename)));
	}


	public static class InputReader {
	    public BufferedReader reader;
	    public StringTokenizer tokenizer;
	
	    public InputReader() {
	        reader = new BufferedReader(new InputStreamReader(System.in), 32768);
	        tokenizer = null;
	    }
	    public InputReader(String filename) throws IOException {
	        reader = new BufferedReader(new FileReader(filename));
	        tokenizer = null;
	    }
	    public String next() {
	        while (tokenizer == null || !tokenizer.hasMoreTokens()) {
	            try {
	                tokenizer = new StringTokenizer(reader.readLine());
	            } catch (IOException e) {
	                throw new RuntimeException(e);
	            }
	        }
	        return tokenizer.nextToken();
	    }
	
	    public int nextInt() {
	        return Integer.parseInt(next());
	    }
	
	}
}

