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

void main(){
	auto ps = readln.chomp.split.map!(to!double);

	printf("%.8lf\r\n", sqrt(pow(ps[2]-ps[0], 2) + pow(ps[3]-ps[1], 2)));
}