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

void main()
{
    auto x4 = readln.chomp.split.map!(to!real).array;
    writefln("%.6f", ((x4[0] - x4[2])^^2 + (x4[1] - x4[3])^^2)^^0.5);
}