#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <utility>
#include <iomanip>
using namespace std;
#define diff(x, y) ((x) > (y) ? ((x) - (y)) : ((y) - (x)))

int main(void) {
	long double Px, Py, Qx, Qy;
	cin >> Px >> Py >> Qx >> Qy;
	cout << diff(Px, Qx) / 2 + diff(Py, Qy) / 2 << endl;
}