#include<iostream>
#include<string>
#include<cstring>
#include<complex>
#include<vector>
#include<algorithm>
#include<cstdlib>
using namespace std;

#define REP(i, j) for (int i = 0; i < (int)j; ++i)
#define FOR(i, j, k) for (int i = (int)j; i < (int)k; ++i)
#define SORT(v) sort(v.begin(), v.end())
#define REVERSE(v) reverse(v.begin(), v.end())
typedef complex<double> P;
typedef vector<int> VI;
typedef long long LL;
typedef pair<int,int> PII;

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	int px, py, qx, qy;
	cin >> px >> py;
	cin >> qx >> qy;

	int dx = abs(px - qx);
	int dy = abs(py - qy);
	string d = to_string((dx + dy) / 2.0);

	cout << d << endl;

	return 0;
}