#include<stdio.h>
int main() {
	int number;
	int sum = 0;
	int count = 0;
	double mean;
	scanf_s("%d", &number);#visual studio中输入为scanf_s
	while (number != -1) {
		sum = sum + number;
		count++;
		scanf_s("%d", &number);
	}
	mean = sum / count;
	printf("%f", mean);
	return 0;
}