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.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class InitialBet {
	public static void main(String[] args) throws IOException {
		BufferedReader bf=new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st=new StringTokenizer(bf.readLine());
		int sum=0;
		for (int i = 0; i <5; i++) {
			sum+=Integer.parseInt(st.nextToken());
		}
		if(sum%5==0 && sum!=0)
			System.out.println(sum/5);
		else
			System.out.println(-1);
	}
	
	
}
