#include <iostream>
#include <cstdio>
#include <vector>
#include <cmath>
#include <cstring>
#include <numeric>
#include <algorithm>
#include <functional>
#include <array>
#include <map>
#include <queue>
#include <limits.h>
#include <set>
#include <stack>

using namespace std;

#define Getsign(n) ((n > 0) - (n < 0))

typedef vector<int> Ivec;
typedef pair<int, int> pii;
int main() {
	pii p, q;

	scanf("%d %d %d %d", &p.first, &p.second, &q.first, &q.second);

	printf("%lf\n", (double)(abs(q.first - p.first) + abs(q.second - p.second))/2);

	return 0;
}