#include <stdio.h>
#include <math.h>

int main() {
  double x1, y1, x2, y2, d;
  scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
  d = sqrt(pow(x1-x2, 2) + pow(y1-y2, 2));
  printf("%lf\n", d);

  return 0;
}