#include <bits/stdc++.h>
#define rep(i, n) for (ll i = 0; i < n; ++i)
typedef long long ll;
using namespace std;
const int INF = 1e9;

int main() {
    int x1, y1, x2, y2;
    cin >> x1 >> y1 >> x2 >> y2;

    cout << (abs(x1 - x2) + abs(y1 - y2)) / 2.0 << endl;

    return 0;
}