#include<bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
using namespace std;

int main() {
    double px,py,qx,qy,ans;
    cin >> px >> py;
    cin >> qx >> qy;
    ans = ( (qx-px) + (qy-py) ) / 2;
    printf("%.1f\n",ans);
}