#include <iostream> #include <stdio.h> #include <algorithm> #include <math.h> #include <utility> #include <vector> using namespace std; int main(void) { int px,py,qx,qy; cin >> px >> py >> qx >> qy; printf("%lf\n",(abs(px-qx)+abs(py-qy))/2.0); return 0; }