#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    int a,b,c,d;
    cin >> a >> b >> c >> d;

    int ans = abs(a - c) / 2 + abs(b - d) / 2;

    if(abs(a-c)%2 == 1 && abs(b-d)%2 == 1){
        cout << ans + 1 << endl;
        return 0;
    }
    if(abs(a-c)%2 == 0 && abs(b-d)%2 == 0){
        cout << ans << endl;
        return 0;
    }

    cout << ans;
    cout << ".5" << endl;
    return 0;
}