"use strict";
main(require("fs").readFileSync("/dev/stdin", "utf8").replace(/\r?\n$/, "").split(/\r?\n/));

function main(lines){
	var p = lines[0].split(" ").map(Number);
	console.log(Math.sqrt(Math.pow(p[0]-p[2],2)+Math.pow(p[1]-p[3],2)));
}