#include<iostream>
#include<string>
#include<algorithm>
#include<cctype>
#include<set>
#include<bitset>
#include<math.h>
#include<map>
#include<queue>
#include<iomanip>
using namespace std;

int main(){
    int px, py, qx, qy;
    cin >> px >> py >> qx >> qy;
    cout << (float)(abs(qx-px) + abs(py - qy))/2 << endl;
}