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.text.*;
import java.math.*;
import java.util.regex.*;

public class apples {

	public static void main(String[] args) {
		Scanner zizo = new Scanner(System.in);
		int c1=zizo.nextInt();
		int c2=zizo.nextInt();
		int c3=zizo.nextInt();
		int c4=zizo.nextInt();
		int c5=zizo.nextInt();
		int sum=c1+c2+c3+c4+c5;
		if(sum%5==0 && sum!=0)System.out.println(sum/5);
		
		else System.out.println(-1);
	}
}