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

int main(){


  double x1,x2,y1,y2,X,Y,ru;

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

  X=x2-x1;
  X=X*X;
  Y=y2-y1;
  Y=Y*Y;

  ru=X+Y;
  printf("%f\n",sqrt(ru));
  return 0;
}