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

int main(void) {
  double x1, x2, y1, y2;
  scanf("%lf %lf %lf %lf", &x1, &y1, &x2, &y2);

  printf("%f\n", sqrt(pow(x2-x1, 2) + pow(y2-y1, 2)));

  return 0;
}


