#include <iostream>
#include <iomanip>
#include <cmath>

using ldouble = long double;

void solve() {
    ldouble px, py, qx, qy;
    std::cin >> px >> py >> qx >> qy;

    ldouble ans = (std::abs(px - qx) + std::abs(py - qy)) / 2;
    std::cout << std::fixed << std::setprecision(1)
              << ans << std::endl;
}

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

    solve();

    return 0;
}