#include <iostream>
#include <vector>
#include <string>
#include <cstdlib>
#include <iomanip>

#define REP(i, a, b) for (int i = int(a); i < int(b); i++)
#define dump(s) cerr << __LINE__ << "\t:" << #s << " = " << (s) << endl
    
using namespace std;

int main () {
    int x[2], y[2];
    REP (i, 0, 2) cin >> x[i] >> y[i];
    cout << fixed << setprecision(20) << (abs(x[1] - x[0]) + abs(y[1] - y[0])) / 2.0 << endl;
    return 0; 
}