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

int main(void){

  double x1, y1, x2, y2;
  double result;

  scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
  result = sqrt ((x1 - x2)*(x1 - x2) +  (y1 - y2)*(y1 - y2));
  printf ("%lf\n", result);

  return 0;

}