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

void main()
{
    auto k = readln.strip.split(" ").map!(to!double).array;
    writeln(sqrt((k[2] - k[0]) * (k[2] - k[0]) + (k[3] - k[1]) * (k[3] - k[1])));
}
