#pragma GCC optimize("Ofast")
#pragma GCC target ("sse4")

#include<iostream>
#include<string>
#include<cstdio>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<ciso646>
#include<random>
#include<map>
#include<set>
#include<bitset>
#include<stack>
#include<unordered_map>
#include<unordered_set>
#include<utility>
#include<cassert>
#include<complex>
#include<numeric>
#include<array>
using namespace std;

//#define int long long
typedef long long ll;

typedef unsigned long long ul;
typedef unsigned int ui;
constexpr ll mod = 1000000007;
const ll INF = mod * mod;
typedef pair<int, int>P;
#define stop char nyaa;cin>>nyaa;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define Rep(i,sta,n) for(int i=sta;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define per1(i,n) for(int i=n;i>=1;i--)
#define Rep1(i,sta,n) for(int i=sta;i<=n;i++)
#define all(v) (v).begin(),(v).end()
typedef pair<ll, ll> LP;
typedef double ld;
typedef pair<ld, ld> LDP;
const ld eps = 1e-12;
const ld pi = acosl(-1.0);

ll mod_pow(ll x, ll n, ll m = mod) {
	ll res = 1;
	while (n) {
		if (n & 1)res = res * x % m;
		x = x * x % m; n >>= 1;
	}
	return res;
}
struct modint {
	ll n;
	modint() :n(0) { ; }
	modint(ll m) :n(m) {
		if (n >= mod)n %= mod;
		else if (n < 0)n = (n % mod + mod) % mod;
	}
	operator int() { return n; }
};
bool operator==(modint a, modint b) { return a.n == b.n; }
modint operator+=(modint& a, modint b) { a.n += b.n; if (a.n >= mod)a.n -= mod; return a; }
modint operator-=(modint& a, modint b) { a.n -= b.n; if (a.n < 0)a.n += mod; return a; }
modint operator*=(modint& a, modint b) { a.n = ((ll)a.n * b.n) % mod; return a; }
modint operator+(modint a, modint b) { return a += b; }
modint operator-(modint a, modint b) { return a -= b; }
modint operator*(modint a, modint b) { return a *= b; }
modint operator^(modint a, ll n) {
	if (n == 0)return modint(1);
	modint res = (a * a) ^ (n / 2);
	if (n % 2)res = res * a;
	return res;
}

ll inv(ll a, ll p) {
	return (a == 1 ? 1 : (1 - p * inv(p % a, a)) / a + p);
}
modint operator/(modint a, modint b) { return a * modint(inv(b, mod)); }

const int max_n = 1 << 17;
modint fact[max_n], factinv[max_n];
void init_f() {
	fact[0] = modint(1);
	for (int i = 0; i < max_n - 1; i++) {
		fact[i + 1] = fact[i] * modint(i + 1);
	}
	factinv[max_n - 1] = modint(1) / fact[max_n - 1];
	for (int i = max_n - 2; i >= 0; i--) {
		factinv[i] = factinv[i + 1] * modint(i + 1);
	}
}
modint comb(int a, int b) {
	if (a < 0 || b < 0 || a < b)return 0;
	return fact[a] * factinv[b] * factinv[a - b];
}

ll calc2(ll x) {
	return x * (x + 1) / 2;
}
void solve() {
	ll n, p, x, y; cin >> n >> p >> x >> y;
	ll ans = -INF;
	auto cs=[&](int restl,int restr,int cl,int cr,int k,ll s,ll ad)->ll {
		if (k == 1)return -INF;
		/*if (restl == 2 && restr == 9 && cl == 0 && cr == 1 && k == 9 && s == 9 && ad == 97) {
			cout << "hello\n";
		}*/
		int xl = restl - cl * k;
		int xr = restr - cr * k;
		ll sum = 0;
		int dl = xl / (k - 1);
		int dr = xr / (k - 1);
		int rl = xl % (k - 1);
		int rr = xr % (k - 1);
		int nl = xl/(k-1); if (xl % (k - 1))nl++;
		int nr = xr/(k-1); if (xr % (k - 1))nr++;
		if (nl + cl > x)return -INF;
		/*if (restl == 2 && restr == 9 && cl == 0 && cr == 1 && k == 9 && s == 9 && ad == 97) {
			cout << "hello\n";
		}*/
		if (nr + cr > p - x)return -INF;
		//[0,x),(x,p]
		ll mi = calc2(cl - 1) + calc2(x + cr) - calc2(x);
		if (mi > s)return -INF;
		ll ma = calc2(x - 1 - nl) - calc2(x - 1 - nl - cl);
		ma += calc2(p - nr) - calc2(p - nr - cr);

		/*if (restl == 2 && restr == 9 && cl == 0 && cr == 1 && k == 9 && s == 9 && ad == 97) {
			cout << "hello\n";
		}*/
		ll b = 0;
		b += (calc2(x - 1) - calc2(x - 1 - dl)) * (k - 1);
		b += (x - 1 - dl) * rl;
		b += (calc2(p) - calc2(p - dr)) * (k - 1);
		b += (p - dr) * rr;
		if (s <= ma) {
			return b + ad;
		}
		else {
			ll sup = calc2(x - 1) - calc2(x - 1-cl);
			sup += calc2(p) - calc2(p - cr);
			//cout << "?? " << sup << "\n";
			if (sup < s)return -INF;
			ll dif = s - ma;
			//cost,number
			vector<LP> v;
			if (rl > 0)v.push_back({ rl,cl });
			if (rr > 0)v.push_back({ rr,cr });
			v.push_back({ k - 1,cl * dl });
			v.push_back({ k - 1,cr * dr });
			sort(all(v));
			rep(i, v.size()) {
				ll z = min(v[i].second, dif);
				dif -= z;
				b -= z * v[i].first;
			}
			return b + ad;
		}
		return -INF;
	};
	if (x == p) {
		if (y == p) {
			cout << n * p << "\n";
		}
		else {
			cout << -1 << "\n";
		}
	}
	else if (x == 0) {
		if (y == 0) {
			cout << n/2 * p << "\n";
		}
		else {
			cout << -1 << "\n";
		}
	}
	else {
		for (int cl = 0; cl <= n / 2; cl++) {
			for (int cr = 0; cr <= n / 2; cr++) {
				if (cl + cr + 1 > n / 2) {
					if (x == y) {
						ll val=cs(n / 2 - cl, n / 2 - cr, 0, 0, cl + cr + 1, 0, (cl + cr + 1) * x);
						ans = max(ans, val);
					}
					continue;
				}
				for (int k = cl + cr + 1; k <= n; k++) {
					if (k != cl + cr + 1 && k > n / 2)continue;
					ll sl = (n / 2 - cl) / k;
					ll sr = (n / 2 - cr) / k;
					for (int le = 0; le <= sl; le++)for (int ri = 0; ri <= sr; ri++) {
						ll num = le + ri;
						if (k == cl + cr + 1)num++;
						if (num == 0)continue;
						
						ll sum = num * y;
						if (k == cl + cr + 1)sum -= x;
						ll z = num * y * k;
						if (k != cl + cr + 1)z += (cl + cr + 1) * x;
						ll val=cs(n / 2 - cl, n / 2 - cr, le, ri, k, sum,z);
						ans = max(ans, val);
						//cout << cl << " " << cr << " " << k << " " << le << " " << ri << " " << val << "\n";
						//cout << sum << " " << z << "\n";
						if (cl == 7 && cr == 0 && k == 9 && le == 0 && ri == 1) {
							//cout << sum << " " << z << "\n";
						}
					}
				}
			}
		}
		if (x >= n / 2 && p - x >= n / 2) {
			ll d = n / 2;
			ll l1 = calc2(d - 1);
			ll r1 = calc2(x - 1) - calc2(x - 1 - d);
			ll l2 = calc2(x + d) - calc2(x);
			ll r2 = calc2(p) - calc2(p - d);
			ll sl = l1 + l2 + x;
			ll sr = r1 + r2 + x;
			if (sl <= n * y && n * y <= sr) {
				ans = max(ans, n * y);
			}
		}
		if (ans == -INF)ans = -1;
		cout << ans << "\n";
	}
}

signed main() {

	ios::sync_with_stdio(false);
	cin.tie(0);
	//cout << fixed << setprecision(15);
	//init_f();
	//init();
	//expr();
	//int t; cin >> t; rep(i, t)
	int t = 10;rep(i,t)
	solve();
	return 0;
}