package adv2020;

import java.io.*;
import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.util.Queue;

// まだ途中
public class Day6 {
	InputStream is;
	FastWriter out;

	// 17 8 1 3
	// 19 17 2 7
	// 19 9 2 9
	String INPUT = "";

	public void solve()
	{
		while(!eof())go();
	}

	// a+(a+1)+..+(a+b-1)
	long sum(long a, long b)
	{
		return a*b+(b-1)*b/2;
	}

	// a+(a-1)+..+(a-b+1)
	long sumr(long a, long b)
	{
		return a*b-(b-1)*b/2;
	}

	void go()
	{
		int n = ni();
		long P = nl(), X = nl(), Y = nl();

		int h = n/2;
		if(X > Y){
			if(n >= 7){
				if(X < P) {
					long ans = -1;
					for(int F = 2;F <= h;F++){
						long S = F*Y;
						int num = h+1-F;
						int hnum = h;
						for(long x = P;x > X && hnum > 0;x--){
							int des = Math.min(hnum, F-1);
							S += x*des;
							hnum -= des;
						}
						if(hnum > F-1)continue;
						num += hnum;

						for(long x = X;x >= 0 && num > 0;x--){
							if(x == Y)continue;
							int des = Math.min(num, F-1);
							S += x*des;
							num -= des;
						}
						if(num > 0)continue;

						ans = Math.max(ans, S);

					}
					out.println(ans);
					return;
				}else{
					out.println(-1);
				}
			}else if(n == 5){
				if(X < P-1){
					out.println(Y+Y+X+P-1+P);
					return;
				}else{
					out.println(-1);
					return;
				}
			}else if(n == 3){
				// [0,Y], X-Y+[X-Y+1,M-Y]
				if(X < P && Math.max(0, X-Y+X-Y+1) <= Math.min(Y, X-Y+P-Y)){
					out.println(3*Y);
					return;
				}else{
					out.println(-1);
					return;
				}
			}else if(n == 1){
				out.println(-1);
				return;
			}
			assert false;
		}else if(X == Y){
			out.println(Y*(h+1) + P*h);
			return;
		}else if(X < Y){
			long ans = -1;
			for(int F = 2;F <= h;F++){
				// XがLに属する場合
				// F*Lしかないのかな？
				{
					int L = (h+1+F-1)/F;
					int rem = n - F*L;
					outer:
					for(int H = 1;F*H <= rem;H++) {
						int hrem = rem - F*H;
						if(hrem < 0)continue;
						int h1 = hrem / (F-1);
						int hr1 = hrem % (F-1);
						int hh = H + h1 + (hr1 > 0 ? 1 : 0);
						if(P-(X+1)+1 < hh)continue;

						long lbal = sum(Y-X, L);
						// H側の最小バランス
						long hbalmin = sum(X+1-Y, H);
						long hbalmax = sumr(P-Y, H);
						// 00000111166666888
						if (hbalmin <= lbal && lbal <= hbalmax){
							long lbalst = lbal;
							long hbalst = sumr(P - Y - (hh - H), H);
							//									tr(F, L, H, lbalmin, lbalmax, lbalst, hbalmin, hbalmax, hbalst);
//							tr(lbalst, hbalst);
							long S = Y * (H + L) * F;
							S += (F - 1) * sumr(P, h1) + hr1 * (P - h1);
							if (lbalst <= hbalst) {
								// 安定な場合
								ans = Math.max(ans, S);
							} else {
								if(hr1 > 0){
									// hr1をH回移動できる
									long hbalst2 = hbalst + H;
									if(lbalst <= hbalst2){
										S -= (lbalst - hbalst) * hr1;
										ans = Math.max(ans, S);
										continue outer;
									}
									hbalst = hbalst2;
								}
								// F-1をたくさん移動できる
								S -= (lbalst - hbalst) * (F-1);
								ans = Math.max(ans, S);
							}
						}
					}
				}

				for(int L = 1;F*L <= h+1;L++){
					for(int H = 1;F*H <= h+1;H++){

						// XがどちらのFでもない場合
						outer:
						{
							int hrem = h - F*H;
							if(hrem < 0)break outer;
							int h1 = hrem / (F-1);
							int hr1 = hrem % (F-1);
							// H側の合計高さ
							int hh = H + h1 + (hr1 > 0 ? 1 : 0);
							if(P-X+1 < hh)break outer;
							if(P-X+1 == hh){
								// やっかい
								tr(F, H, L);
							}else{
								int lrem = h+1 - F*L;
								if(lrem < 0)break outer;
								int l1 = lrem / (F-1);
								int lr1 = lrem % (F-1);
								// L側の合計高さ
								int lh = L + l1 + (lr1 > 0 ? 1 : 0);
								if(X-0+1 < lh)break outer;

								// L側の最小バランス
								long lbalmin = sum(Y-X, L);
								long lbalmax = sumr(Y, L);

								// H側の最小バランス
								long hbalmin = sum(X+1-Y, H);
								long hbalmax = sumr(P-Y, H);
//								tr(lbalmin, lbalmax, hbalmin, hbalmax);
//								tr(F, L, H, lh, hh, l1, h1);
								// 00000111166666888
								if (Math.max(lbalmin, hbalmin) <= Math.min(lbalmax, hbalmax)) {
									long lbalst = sum(Y-X+(lh-L), L);
									long hbalst = sumr(P-Y-(hh-H), H);
//									tr(F, L, H, lbalmin, lbalmax, lbalst, hbalmin, hbalmax, hbalst);
									if(lbalst <= hbalst){
										// 安定な場合
										long S = Y*(H+L)*F;
										S += (F-1)*sumr(X, l1) + lr1*(X-l1);
										S += (F-1)*sumr(P, h1) + hr1*(P-h1);
										ans = Math.max(ans, S);
									}else {
//																				tr(lbalmin, lbalmax, lbalst, hbalmin, hbalmax, hbalst);
										tr("YABAI", F, L, H);
									}
								}else{
									break outer;
								}
							}
						}
					}
				}

				inner:
				{
					long S = Y*F;
					int lrem = h+1;
					for(long t = X;t >= 0 && lrem > 0;t--){
						int de = Math.min(lrem, F-1);
						S += t*de;
						lrem -= de;
					}
					if(lrem > 0)break inner;

					int hrem = h-F;
					for(long t = P;t >= Y+1 && hrem > 0;t--){
						int de = Math.min(hrem, F-1);
						S += t*de;
						hrem -= de;
					}
					if(hrem > 0)break inner;
					ans = Math.max(ans, S);
				}
			}

			// F = 1
			if(X-h >= 0 && X+h <= P){
				// X-h X-h+1 ... X
				long lmax = X*(X+1)/2 - (X-h-1)*(X-h)/2;
				long lmin = X + h*(h-1)/2;

				long hmax = P*(P+1)/2 - (P-h+1)*(P-h)/2;
				long hmin = (X+h)*(X+h+1)/2 - X*(X+1)/2;
//				tr(lmin, lmax, hmin, hmax);

				if(lmin + hmin <= n*Y && n*Y <= lmax + hmax) {
					ans = Math.max(ans, n * Y);
				}
			}

			out.println(ans);
			return;
		}
	}

	public static void main(String[] args) {
		new Day6().run();
	}

	public void run()
	{
		long S = System.currentTimeMillis();
		is = INPUT.isEmpty() ? System.in : new ByteArrayInputStream(INPUT.getBytes());
		out = new FastWriter(System.out);

		solve();
		out.flush();
		long G = System.currentTimeMillis();
		tr(G-S+"ms");
		//		Thread t = new Thread(null, null, "~", Runtime.getRuntime().maxMemory()){
		//			@Override
		//			public void run() {
		//				long s = System.currentTimeMillis();
		//				solve();
		//				out.flush();
		//				if(!INPUT.isEmpty())tr(System.currentTimeMillis()-s+"ms");
		//			}
		//		};
		//		t.start();
		//		t.join();
	}

	private boolean eof()
	{
		if(lenbuf == -1)return true;
		int lptr = ptrbuf;
		while(lptr < lenbuf)if(!isSpaceChar(inbuf[lptr++]))return false;

		try {
			is.mark(1000);
			while(true){
				int b = is.read();
				if(b == -1){
					is.reset();
					return true;
				}else if(!isSpaceChar(b)){
					is.reset();
					return false;
				}
			}
		} catch (IOException e) {
			return true;
		}
	}

	private final byte[] inbuf = new byte[1024];
	public int lenbuf = 0, ptrbuf = 0;

	private int readByte()
	{
		if(lenbuf == -1)throw new InputMismatchException();
		if(ptrbuf >= lenbuf){
			ptrbuf = 0;
			try { lenbuf = is.read(inbuf); } catch (IOException e) { throw new InputMismatchException(); }
			if(lenbuf <= 0)return -1;
		}
		return inbuf[ptrbuf++];
	}

	private boolean isSpaceChar(int c) { return !(c >= 33 && c <= 126); }
	//	private boolean isSpaceChar(int c) { return !(c >= 32 && c <= 126); }
	private int skip() { int b; while((b = readByte()) != -1 && isSpaceChar(b)); return b; }

	private double nd() { return Double.parseDouble(ns()); }
	private char nc() { return (char)skip(); }

	private String ns()
	{
		int b = skip();
		StringBuilder sb = new StringBuilder();
		while(!(isSpaceChar(b))){
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}

	private char[] ns(int n)
	{
		char[] buf = new char[n];
		int b = skip(), p = 0;
		while(p < n && !(isSpaceChar(b))){
			buf[p++] = (char)b;
			b = readByte();
		}
		return n == p ? buf : Arrays.copyOf(buf, p);
	}

	private char[][] nm(int n, int m)
	{
		char[][] map = new char[n][];
		for(int i = 0;i < n;i++)map[i] = ns(m);
		return map;
	}

	private int[] na(int n)
	{
		int[] a = new int[n];
		for(int i = 0;i < n;i++)a[i] = ni();
		return a;
	}

	private long[] nal(int n)
	{
		long[] a = new long[n];
		for(int i = 0;i < n;i++)a[i] = nl();
		return a;
	}

	private int ni()
	{
		int num = 0, b;
		boolean minus = false;
		while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'));
		if(b == '-'){
			minus = true;
			b = readByte();
		}

		while(true){
			if(b >= '0' && b <= '9'){
				num = num * 10 + (b - '0');
			}else{
				return minus ? -num : num;
			}
			b = readByte();
		}
	}

	private long nl()
	{
		long num = 0;
		int b;
		boolean minus = false;
		while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'));
		if(b == '-'){
			minus = true;
			b = readByte();
		}

		while(true){
			if(b >= '0' && b <= '9'){
				num = num * 10 + (b - '0');
			}else{
				return minus ? -num : num;
			}
			b = readByte();
		}
	}

	public static class FastWriter
	{
		private static final int BUF_SIZE = 1<<13;
		private final byte[] buf = new byte[BUF_SIZE];
		private final OutputStream out;
		private int ptr = 0;

		private FastWriter(){out = null;}

		public FastWriter(OutputStream os)
		{
			this.out = os;
		}

		public FastWriter(String path)
		{
			try {
				this.out = new FileOutputStream(path);
			} catch (FileNotFoundException e) {
				throw new RuntimeException("FastWriter");
			}
		}

		public FastWriter write(byte b)
		{
			buf[ptr++] = b;
			if(ptr == BUF_SIZE)innerflush();
			return this;
		}

		public FastWriter write(char c)
		{
			return write((byte)c);
		}

		public FastWriter write(char[] s)
		{
			for(char c : s){
				buf[ptr++] = (byte)c;
				if(ptr == BUF_SIZE)innerflush();
			}
			return this;
		}

		public FastWriter write(String s)
		{
			s.chars().forEach(c -> {
				buf[ptr++] = (byte)c;
				if(ptr == BUF_SIZE)innerflush();
			});
			return this;
		}

		private static int countDigits(int l) {
			if (l >= 1000000000) return 10;
			if (l >= 100000000) return 9;
			if (l >= 10000000) return 8;
			if (l >= 1000000) return 7;
			if (l >= 100000) return 6;
			if (l >= 10000) return 5;
			if (l >= 1000) return 4;
			if (l >= 100) return 3;
			if (l >= 10) return 2;
			return 1;
		}

		public FastWriter write(int x)
		{
			if(x == Integer.MIN_VALUE){
				return write((long)x);
			}
			if(ptr + 12 >= BUF_SIZE)innerflush();
			if(x < 0){
				write((byte)'-');
				x = -x;
			}
			int d = countDigits(x);
			for(int i = ptr + d - 1;i >= ptr;i--){
				buf[i] = (byte)('0'+x%10);
				x /= 10;
			}
			ptr += d;
			return this;
		}

		private static int countDigits(long l) {
			if (l >= 1000000000000000000L) return 19;
			if (l >= 100000000000000000L) return 18;
			if (l >= 10000000000000000L) return 17;
			if (l >= 1000000000000000L) return 16;
			if (l >= 100000000000000L) return 15;
			if (l >= 10000000000000L) return 14;
			if (l >= 1000000000000L) return 13;
			if (l >= 100000000000L) return 12;
			if (l >= 10000000000L) return 11;
			if (l >= 1000000000L) return 10;
			if (l >= 100000000L) return 9;
			if (l >= 10000000L) return 8;
			if (l >= 1000000L) return 7;
			if (l >= 100000L) return 6;
			if (l >= 10000L) return 5;
			if (l >= 1000L) return 4;
			if (l >= 100L) return 3;
			if (l >= 10L) return 2;
			return 1;
		}

		public FastWriter write(long x)
		{
			if(x == Long.MIN_VALUE){
				return write("" + x);
			}
			if(ptr + 21 >= BUF_SIZE)innerflush();
			if(x < 0){
				write((byte)'-');
				x = -x;
			}
			int d = countDigits(x);
			for(int i = ptr + d - 1;i >= ptr;i--){
				buf[i] = (byte)('0'+x%10);
				x /= 10;
			}
			ptr += d;
			return this;
		}

		public FastWriter write(double x, int precision)
		{
			if(x < 0){
				write('-');
				x = -x;
			}
			x += Math.pow(10, -precision)/2;
			//		if(x < 0){ x = 0; }
			write((long)x).write(".");
			x -= (long)x;
			for(int i = 0;i < precision;i++){
				x *= 10;
				write((char)('0'+(int)x));
				x -= (int)x;
			}
			return this;
		}

		public FastWriter writeln(char c){
			return write(c).writeln();
		}

		public FastWriter writeln(int x){
			return write(x).writeln();
		}

		public FastWriter writeln(long x){
			return write(x).writeln();
		}

		public FastWriter writeln(double x, int precision){
			return write(x, precision).writeln();
		}

		public FastWriter write(int... xs)
		{
			boolean first = true;
			for(int x : xs) {
				if (!first) write(' ');
				first = false;
				write(x);
			}
			return this;
		}

		public FastWriter write(long... xs)
		{
			boolean first = true;
			for(long x : xs) {
				if (!first) write(' ');
				first = false;
				write(x);
			}
			return this;
		}

		public FastWriter writeln()
		{
			return write((byte)'\n');
		}

		public FastWriter writeln(int... xs)
		{
			return write(xs).writeln();
		}

		public FastWriter writeln(long... xs)
		{
			return write(xs).writeln();
		}

		public FastWriter writeln(char[] line)
		{
			return write(line).writeln();
		}

		public FastWriter writeln(char[]... map)
		{
			for(char[] line : map)write(line).writeln();
			return this;
		}

		public FastWriter writeln(String s)
		{
			return write(s).writeln();
		}

		private void innerflush()
		{
			try {
				out.write(buf, 0, ptr);
				ptr = 0;
			} catch (IOException e) {
				throw new RuntimeException("innerflush");
			}
		}

		public void flush()
		{
			innerflush();
			try {
				out.flush();
			} catch (IOException e) {
				throw new RuntimeException("flush");
			}
		}

		public FastWriter print(byte b) { return write(b); }
		public FastWriter print(char c) { return write(c); }
		public FastWriter print(char[] s) { return write(s); }
		public FastWriter print(String s) { return write(s); }
		public FastWriter print(int x) { return write(x); }
		public FastWriter print(long x) { return write(x); }
		public FastWriter print(double x, int precision) { return write(x, precision); }
		public FastWriter println(char c){ return writeln(c); }
		public FastWriter println(int x){ return writeln(x); }
		public FastWriter println(long x){ return writeln(x); }
		public FastWriter println(double x, int precision){ return writeln(x, precision); }
		public FastWriter print(int... xs) { return write(xs); }
		public FastWriter print(long... xs) { return write(xs); }
		public FastWriter println(int... xs) { return writeln(xs); }
		public FastWriter println(long... xs) { return writeln(xs); }
		public FastWriter println(char[] line) { return writeln(line); }
		public FastWriter println(char[]... map) { return writeln(map); }
		public FastWriter println(String s) { return writeln(s); }
		public FastWriter println() { return writeln(); }
	}

	public static void trnz(int... o)
	{
		for(int i = 0;i < o.length;i++)if(o[i] != 0)System.out.print(i+":"+o[i]+" ");
		System.out.println();
	}

	// print ids which are 1
	public static void trt(long... o)
	{
		Queue<Integer> stands = new ArrayDeque<>();
		for(int i = 0;i < o.length;i++){
			for(long x = o[i];x != 0;x &= x-1)stands.add(i<<6|Long.numberOfTrailingZeros(x));
		}
		System.out.println(stands);
	}

	public static void tf(boolean... r)
	{
		for(boolean x : r)System.out.print(x?'#':'.');
		System.out.println();
	}

	public static void tf(boolean[]... b)
	{
		for(boolean[] r : b) {
			for(boolean x : r)System.out.print(x?'#':'.');
			System.out.println();
		}
		System.out.println();
	}

	public void tf(long[]... b)
	{
		if(INPUT.length() != 0) {
			for (long[] r : b) {
				for (long x : r) {
					for (int i = 0; i < 64; i++) {
						System.out.print(x << ~i < 0 ? '#' : '.');
					}
				}
				System.out.println();
			}
			System.out.println();
		}
	}

	public void tf(long... b)
	{
		if(INPUT.length() != 0) {
			for (long x : b) {
				for (int i = 0; i < 64; i++) {
					System.out.print(x << ~i < 0 ? '#' : '.');
				}
			}
			System.out.println();
		}
	}

	private void tr(Object... o) { if(INPUT.length() != 0)System.out.println(Arrays.deepToString(o)); }
}