import std.stdio, std.string, std.conv;
import std.math;

void main() {
    auto pt = getStdin!(string[])[0].split(" ").to!(double[]);
	double dist = sqrt(pow(pt[2] - pt[0], 2) + pow(pt[3] - pt[1], 2));
	"%.8f".writefln(dist);
}

T getStdin(T)() {
    string[] cmd;
    string line;
    while ((line = chomp(stdin.readln())) != "") cmd ~= line;
    return to!(T)(cmd);
}