#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
using namespace std;
using ll = long long ;
using P = pair<int,int> ;
using pll = pair<long long,long long>;
constexpr int INF = 1e9;
constexpr long long LINF = 1e17;
constexpr int MOD = 1000000007;
constexpr double PI = 3.14159265358979323846;

int main(){
    P a,b;
    cin >> a.first >> a.second;
    cin >> b.first >> b.second;
    int k = abs(a.first-b.first) + abs(a.second-b.second);
    printf("%.2f\n",k/2.0);
    return 0;
}