#include <cstdio>
#include <cmath>
#include <algorithm>

using namespace std;

typedef long double ld;

int xx1, yy1, xx2, yy2;

ld hyp(ld x, ld y) {
  return sqrt(x*x+y*y);
}

ld f(ld y) {
  return hyp(xx1, yy1-y)+hyp(xx2, yy2-y);
}

int main(void) {
  scanf("%d%d%d%d", &xx1, &yy1, &xx2, &yy2);
  ld left = min(yy1, yy2);
  ld right = max(yy1, yy2);
	for (int loop = 0; loop < 10000; ++loop){
		if (f((left * 2 + right) / 3) < f((left + right * 2) / 3)){
			right = (left + right * 2) / 3;
		} else {
			left = (left * 2 + right) / 3;
		}
	}
  printf("%.10Lf\n", (left+right)/2.0);
  return 0;
}