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

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

  auto ans = sqrt(pow(p[2] - p[0], 2) + pow(p[3] - p[1], 2));
  printf("%.8lf\n", ans);
}