import java.io.BufferedReader;
import java.io.InputStreamReader;
public class Solution {
public static void main(String[] args) throws Exception {
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
double sum = 0;
double i = 0;
while (true) {
String sNumber = reader.readLine();
double number = Double.parseDouble(sNumber);
if (number != -1) {
sum = sum + number;
i++;
} else if (number == -1)
break;
}
System.out.println(sum / i);
}
}
import java.io.BufferedReader;
import java.io.InputStreamReader;
public class Solution {
public static void main(String[] args) throws Exception {
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
double sum = 0;
double i = 0;
while (true) {
String sNumber = reader.readLine();
double number = Double.parseDouble(sNumber);
if (number != -1) {
sum = sum + number;
i++;
} else if (number == -1)
break;
}
System.out.println(sum / i);
}
}