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.Scanner;

public class Main{
	public static void main(String[] args){
		Scanner input = new Scanner(System.in);
		int[] koinAkhir = new int[5];
		for(int i = 0; i < koinAkhir.length; i++){
			koinAkhir[i] = input.nextInt();
		}
		
		double min = koinAkhir[0];
		double max = koinAkhir[0];
		
		for(int j = 0; j < koinAkhir.length; j++){
			if(koinAkhir[j] > max) max = koinAkhir[j];
			else if(koinAkhir[j] < min) min = koinAkhir[j];
		}
		
		int middle = (int)min + (int)Math.ceil((max - min) / 2);
		
		int positive = 0;
		int negative = 0;
		
		for(int k = 0; k < koinAkhir.length; k++){
			if(middle - koinAkhir[k] > 0) positive += middle - koinAkhir[k];
			else if(middle - koinAkhir[k] < 0) negative += middle - koinAkhir[k];
		}
		
		if(positive + negative != 0){
			positive = 0;
			negative = 0;
			middle -= 1;
			for(int l = 0; l < koinAkhir.length; l++){
				if(middle - koinAkhir[l] > 0) positive += middle - koinAkhir[l];
				else if(middle - koinAkhir[l] < 0) negative += middle - koinAkhir[l];
			}
			if(positive + negative != 0) middle = -1;
			}
		
		System.out.println(middle);
	}
}
		 		  		   					 					 						