#include <iostream>
using namespace std;
void SR(double a, double b, double c, double d) {
cout << (a + b + c + d) / 4 << endl;
}
signed main() {
double a, b, c, d;
cin >> a >> b >> c >> d;
SR(a, b, c, d);
return 0;
#include <iostream>
using namespace std;
void SR(double a, double b, double c, double d) {
cout << (a + b + c + d) / 4 << endl;
}
signed main() {
double a, b, c, d;
cin >> a >> b >> c >> d;
SR(a, b, c, d);
return 0;
}