#include <iostream>
using namespace std;

int main() {
    int p_x, p_y, q_x, q_y;
    cin >> p_x >> p_y;
    cin >> q_x >> q_y;
    double ans;
    ans = double(abs(p_x - q_x) + abs(p_y - q_y)) / 2.0;
    printf("%f\n", ans);
    return 0;
}