#include #include #include #include #include #include #include #include #include #include #include #include #define FOR(i, a, b) for(int i=(a);i<=(b);i++) #define RFOR(i, a, b) for(int i=(a);i>=(b);i--) #define MOD 1000000007 #define INF 1000000000 using namespace std; int main(void) { int px, py, qx, qy; int x, y; cin >> px >> py >> qx >> qy; x = abs(px - qx); y = abs(py - qy); cout << (x + y) / 2.0 << endl; return 0; }