#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;i++)
#define REP(i,a,b) for(int i=a;i<b;i++)
using ll = long long;
using ull = unsigned long long;
using namespace std;

double px,py;
double qx,qy;

void solve()
{
    cin >> px >> py >> qx >> qy;
    cout << fixed << setprecision(1) << (abs(px-qx) / 2.0 + abs(py-qy) / 2.0) << endl;
}

int main(){cin.tie(0);ios_base::sync_with_stdio(false);solve();return 0;}