#include<stdio.h>
#include<math.h>
int main(){
  double x1,x2,y1,y2;
  double len;

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

  len = sqrt(x2*x2 + y2*y2);
  printf("%.4f\n",len);
  return 0;
}

