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

int main(){

  double x1,y1,x2,y2;

  double distance;

  scanf("%lf %lf %lf %lf",&x1,&y1,&x2,&y2);

  distance=sqrt(pow((x2-x1),2)+pow((y2-y1),2));

  printf("%f",distance);

  return 0;

}