// tested by Hightail - https://github.com/dj3500/hightail
import std.stdio, std.string, std.conv, std.algorithm;
import std.range, std.array, std.math, std.typecons, std.container, core.bitop;

immutable inf = 10^^9 + 7;
int xa, ya;
int xb, yb;

void main() {
    scan(xa, ya);
    scan(xb, yb);

    if (ya == yb) {
        writeln(ya);
        return;
    }

    real top, btm;

    if (ya > yb) {
        swap(xa, xb);
        swap(ya, yb);
    }

    top = yb;
    btm = ya;

    foreach (i ; 0 .. 1000) {
        real m1 = btm + (top - btm) / 3;
        real m2 = btm + (top - btm) * 2 / 3;

        debug {
            writeln(m1, " ", m2);
        }

        real d1 = sqrt(xa^^2 + (m1 - ya)^^2) + sqrt(xb^^2 + (yb - m1)^^2);
        real d2 = sqrt(xa^^2 + (m2 - ya)^^2) + sqrt(xb^^2 + (yb - m2)^^2);

        if (d1 < d2) {
            top = m2;
        }
        else {
            btm = m1;
        }
    }

    writefln("%.10f", btm);
}

void scan(T...)(ref T args) {
    string[] line = readln.split;
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}