problem

I want to put as many rectangular tiles as possible on a rectangular wall with a size of $ h $ in height and $ w $ in width, and a size of $ a $ in height and $ b $ in width.

The following conditions must be met when attaching tiles.


* Do not stack tiles.
* Do not apply tiles diagonally, that is, any edge of the tile is parallel or perpendicular to any edge of the wall.
* Do not change the orientation of the tiles, that is, do not swap the vertical and horizontal directions.



When as many tiles as possible are pasted, find the sum of the areas not covered by the tiles.



output

Output the total area of ​​the part not covered by the tile. Also, output a line break at the end.

Example

Input

5 8
2 2


Output

8
// vvvvvvvvvvvv TEMPLATE vvvvvvvvvvvv
#include <bits/stdc++.h>
using namespace std; using ll = long long; using P = pair<ll, ll>;
const ll linf = 1e18; const double eps = 1e-12, pi = acos(-1);
#define FOR(i,a,b) for (ll i=(a),__last_##i=(b);i<__last_##i;i++)
#define RFOR(i,a,b) for (ll i=(b)-1,__last_##i=(a);i>=__last_##i;i--)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) RFOR(i,0,n)
#define __GET_MACRO3(_1, _2, _3, NAME, ...) NAME
#define each(i,a) for (auto&& i : a)
#define rep(...) __GET_MACRO3(__VA_ARGS__, FOR, REP)(__VA_ARGS__)
#define rrep(...) __GET_MACRO3(__VA_ARGS__, RFOR, RREP)(__VA_ARGS__)
#define pb push_back
#define eb emplace_back
#define all(a) begin(a),end(a)
#define chmin(x,v) x = min(x, v)
#define chmax(x,v) x = max(x, v)
template<typename T> ll min(ll a, const T&& b) { return a < b ? a : b; }
template<typename T> ll min(const T&& a, ll b) { return a < b ? a : b; }
template<typename T> ll max(ll a, const T&& b) { return a > b ? a : b; }
template<typename T> ll max(const T&& a, ll b) { return a > b ? a : b; }
template<typename Head> void out(Head h) { cout << h << endl; } template<typename Head, typename... Tail>void out(Head h, Tail... t) { cout << h << " "; out(t...); }
template<typename T> istream& operator>>(istream& is, vector<T>& v) { each(x,v) is >> x; return is; }
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v) { rep(i,v.size()) { if (i) os << " "; os << v[i]; } return os; }
ostream& operator<<(ostream& os, const vector<string>& v) { rep(i,v.size()) { if (i) os << endl; os << v[i]; } return os; }
template<typename T> ostream& operator<<(ostream& os, const vector<vector<T>>& v) { rep(i,v.size()) { if (i) os << endl; os << v[i]; } return os; }
template<typename T1, typename T2> ostream& operator<<(ostream& os, const pair<T1, T2>& p) { return os << "(" << p.first << ", " << p.second << ")"; }
struct yes_no : std::numpunct<char> { string_type do_truename() const { return "Yes"; } string_type do_falsename() const { return "No"; } };
void solve(); int main() {
  ios::sync_with_stdio(false); cin.tie(0); locale loc(locale(), new yes_no); cout.imbue(loc); cout << fixed << setprecision(10) << boolalpha;
  solve();
}
// ^^^^^^^^^^^^ TEMPLATE ^^^^^^^^^^^^

void solve() {
  ll h, w, a, b; cin >> h >> w >> a >> b;
  cout << w * h - (a * (h / a)) * (b * (w / b)) << endl;
}

