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

int main(void){
  double x1, y1, x2, y2;
  
  scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);
  double diffx = pow(x1 - x2, 2.0);
  double diffy = pow(y1 - y2, 2.0);
  double distance = sqrt(diffx + diffy);

  printf("%lf\n", distance);
  return 0;
}