#include <bits/stdc++.h>
using namespace std;
typedef pair<int,int> pii;
typedef long long ll;
#define ITR(v,c) for(auto v=begin(c);v!=end(c);v++)
#define FOR(v,a,n) for(int v=a;v<(int)(n);v++)
#define FORE(x,arr) for(auto &x:arr)
#define REP(v,n) FOR(v,0,n)
#define ALL(c) begin(c),end(c)
const int DX[4]={0,1,0,-1}, DY[4]={-1,0,1,0};
const int INF = 1e9;
template<class T,class U>ostream&operator<<(ostream &os,const pair<T,U> &p){
    os<<"("<<p.first<<","<<p.second<<")";return os;}
template<class T>ostream&operator<<(ostream &os,const vector<T> &v){
    ITR(i,v)os<<*i<<(i==end(v)-1?"":"\n");return os;}
//------------------------------------------------------------------------------

int main(int argc, char const *argv[]) {
    ll px,py,qx,qy;
    cin>>px>>py>>qx>>qy;
    printf("%f\n",(abs(px-qx)+abs(py-qy))/2.0);
    return 0;
}