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

T read(T)() { return readln.chomp.to!T; }
T[] reads(T)() { return readln.split.to!(T[]); }
alias readint = read!int;
alias readints = reads!int;

alias Point = Tuple!(double, "x", double, "y");

void main() {
    auto xs = reads!double;

    auto a = Point(xs[0], xs[1]);
    auto b = Point(xs[2], xs[3]);

    auto d = sqrt((a.x - b.x) ^^ 2 + (a.y - b.y) ^^ 2);
    writeln(d);
}
