#include <iostream>
#include <string>
using namespace std;

int main()
{
	double ax, ay, bx, by;
	double p;

	cin >> ax >> ay;
	cin >> bx >> by;

	if (ax == bx && ay == by) {
		cout << ay << endl;
		return 0;
	}
	else if (ax == bx) {
		cout << (ay + by) / 2 << endl;
		return 0;
	}
	else if (ay == by) {
		cout << ay << endl;
		return 0;
	}
	else {
		double min = -1;
		cout << -1 << endl;
		return 0;
	}



	return 0;
}