#include <bits/stdc++.h>
using ll = long long;
using llu = long long unsigned;

using namespace std;

int main() {
    ll Px, Py, Qx, Qy;
    cin >> Px >> Py >> Qx >> Qy;
    ll result = (abs(Px-Qx) + abs(Py-Qy));
    long double ret = static_cast<long double>(result/2.0);
    printf("%llf\n", ret);
    return 0;
}