#include #define FOR(i,a,b) for(int i = (a); i < (b); ++i) #define REP(i,n) FOR(i,0,n) #define SZ(n) (int)(n).size() #define ALL(n) (n).begin(), (n).end() using namespace std; typedef long long LL; typedef vector VI; int main() { int px, py, qx, qy; cin >> px >> py >> qx >> qy; double ans = abs(px - qx) + abs(py - qy); cout << ans / 2 << endl; return 0; }