#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int,int>;
#define rep(i,n) for(int i=0;i<n;i++)

const ll mod = ll(1e9) + 7;
const int INF = 1<<29;

int main(){
    int p1,p2,q1,q2;
    cin >> p1 >> p2 >> q1 >> q2;
    double ans = abs(p1-q1) / 2.0 + abs(p2-q2) / 2.0;
    printf("%lf",ans);
    return 0;
}