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

int main(void){

  double x1,x2,y1,y2;
  double dis;

  scanf("%f %f %f %f",x1,y1,x2,y2);
  dis = ((x2 - x1) * (x2 - x1)) + ((y2 - y1) * (y2 - y1));
  dis = sqrt(dis);
  printf("%f",dis);
  return 0;
}