import std.algorithm;
import std.array;
import std.conv;
import std.math;
import std.stdio;
import std.string;
import std.range;

int readint() {
    return readln.chomp.to!int;
}

int[] readints() {
    return readln.split.map!(to!int).array;
}

double distance(double x1, double y1, double x2, double y2) {
    double sq(double n) {
        return n * n;
    }

    return sqrt(sq(x1 - x2) + sq(y1 - y2));
}

void main() {
    auto xs = readln.split.map!(to!double).array;

    auto ans = distance(xs[0], xs[1], xs[2], xs[3]);
    writefln("%.6f", ans);
}