#include<bits/stdc++.h>
#define rep(i, x) for(ll i = 0; i < x; i++)
#define rep2(i, x) for(ll i = 1; i <= x; i++)
#define all(a) (a).begin(),(a).end()
using ll = long long;
using ld = long double;
const ll INF = 10000000000000000;
using namespace std;
signed main() {
    ld a, b, c, d; cin >> a >> b >> c >> d;
	cout << (abs(a - c) + abs(b - d)) / 2 << endl;
	return 0;
}