// macros
#include <iostream>
#include <iomanip>
#include <algorithm>
#include <vector>
#include <string>
#include <map>

using namespace std;
typedef long long ll;
typedef pair<int,int> pii;

#define LLINF 1000000000000000000LL
#define FOR(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define debug(a) cout << "L(" << __LINE__ << "):" << #a << ":" << a << endl;

// problems limits
#define MAX_N 100000

// Variables
int P[2], Q[2];

// problems input
void input()
{
    cin >> P[0] >> P[1];
    cin >> Q[0] >> Q[1];
}

// problems main
void solve()
{
    int df, dy;
    df = abs(Q[0] - P[0]), dy = abs(Q[1] - P[1]);
    double d = df + dy;
    double ans = d/2;
    cout << fixed << setprecision(1) << ans << endl;
}

int main()
{
    ios::sync_with_stdio(false);

    input();

    solve();
}