#include <iostream>
#include <math.h>

double pyt(double sa, double sb)
{
    return ::sqrt((sa * sa) + (sb * sb));
}

double f(double ax, double ay, double bx, double by)
{
    if (ay == by) {
        return ay;
    }

    double p_min = ay < by? ay: by;
    double p_len = ::fabs(ay - by);
    double p_max = p_min + p_len;
    double n  = 0;
    double rc = ax;
    double wk = pyt(ax, p_len) + bx;
    while (n <= p_len) {
        double a = pyt(ax, (ay < by? n: (p_len - n)));
        double b = pyt(bx, (by < ay? n: (p_len - n)));
        double w = a + b;
        if (w < wk) {
            wk = w;
            rc = n;
        }
        n += 0.1;
    }
    return p_min + rc;
}

int main()
{
    double ax, ay, bx, by;
    std::cin >> ax;
    std::cin >> ay;
    std::cin >> bx;
    std::cin >> by;
    std::cout << f(ax, ay, bx, by) << std::endl;
    return 0;
}