#include using namespace std; using ll = long long; using VI = vector; using VL = vector; using PII = std::pair; using PLL = std::pair; #define rep(i, n) for (int i = 0; i < (int)(n); i++) #define repr(i, n) for (int i = (int)(n)-1; i >= 0; i--) #define rep2(i, s, n) for (int i = (s); i < (int)(n); i++) #define allpt(v) (v).begin(), (v).end() #define allpt_r(v) (v).rbegin(), (v).rend() const int mod = 1e9 + 7; const string wsp = " "; const string tb = "\t"; const string rt = "\n"; template void show1dvec(vector v) { int n = v.size() - 1; rep(i, n) cout << v[i] << wsp; cout << v[n] << rt; return; } template void show2dvec(vector> v) { int n = v.size(); rep(i, n) show1dvec(v[i]); } int main() { #ifdef DEBUG cout << "DEBUG MODE" << endl; ifstream in("input.txt"); //for debug cin.rdbuf(in.rdbuf()); //for debug #endif double a, b, c, d; cin >> a >> b >> c >> d; cout << abs(a-c)/2 + abs(b-d)/2 <