The on-board computer on Polycarp's car measured that the car speed at the beginning of some section of the path equals v1 meters per second, and in the end it is v2 meters per second. We know that this section of the route took exactly t seconds to pass.

Assuming that at each of the seconds the speed is constant, and between seconds the speed can change at most by d meters per second in absolute value (i.e., the difference in the speed of any two adjacent seconds does not exceed d in absolute value), find the maximum possible length of the path section in meters.

Input

The first line contains two integers v1 and v2 (1 ≤ v1, v2 ≤ 100) — the speeds in meters per second at the beginning of the segment and at the end of the segment, respectively.

The second line contains two integers t (2 ≤ t ≤ 100) — the time when the car moves along the segment in seconds, d (0 ≤ d ≤ 10) — the maximum value of the speed change between adjacent seconds.

It is guaranteed that there is a way to complete the segment so that:

  * the speed in the first second equals v1, 
  * the speed in the last second equals v2, 
  * the absolute value of difference of speeds between any two adjacent seconds doesn't exceed d. 

Output

Print the maximum possible length of the path segment in meters. 

Examples

Input

5 6
4 2


Output

26

Input

10 10
10 0


Output

100

Note

In the first sample the sequence of speeds of Polycarpus' car can look as follows: 5, 7, 8, 6. Thus, the total path is 5 + 7 + 8 + 6 = 26 meters.

In the second sample, as d = 0, the car covers the whole segment at constant speed v = 10. In t = 10 seconds it covers the distance of 100 meters.
import java.util.*;
import java.io.*;
import java.lang.*;
	
public class codes_3
	{
		
		public static void main(String[] args)
		{
			InputReader in = new InputReader(System.in);
			PrintWriter pw = new PrintWriter(System.out);
			
			//Code starts..
			
		
			
			
			int v1 = in.nextInt();
			int v2 = in.nextInt();
			int t = in.nextInt();
			int d = in.nextInt();
			
			int[] t1 = new int[t];
			int[] t2 = new int[t];
			t1[0] = v1;
			t2[t-1] = v2;
			for(int i=1; i<t; i++)
			{
				t1[i] = t1[i-1] + d;
				t2[t-i-1] = t2[t-i] + d;
			}
			
			int ans = 0;
			for(int i=0; i<t; i++)
				ans += Math.min(t1[i], t2[i]);
			pw.print(ans);
			
			
			
			
			//Code ends....
			
			pw.flush();
			pw.close();
		}
	
		
		static class InputReader 
		{
			 
			private final InputStream stream;
			private final byte[] buf = new byte[8192];
			private int curChar, snumChars;
			private SpaceCharFilter filter;
	 
			public InputReader(InputStream stream) 
			{
				this.stream = stream;
			}
	 
			public int snext() 
			{
				if (snumChars == -1)
					throw new InputMismatchException();
				if (curChar >= snumChars) 
				{
					curChar = 0;
					try 
					{
						snumChars = stream.read(buf);
					} 
					catch (IOException e) 
					{
						throw new InputMismatchException();
					}
					if (snumChars <= 0)
						return -1;
				}
				return buf[curChar++];
			}
	 
			public int nextInt() 
		    {
				int c = snext();
				while (isSpaceChar(c)) 
				{
					c = snext();
				}
				int sgn = 1;
				if (c == '-')
			    {
					sgn = -1;
					c = snext();
				}
				int res = 0;
				do 
				{
					if (c < '0' || c > '9')
						throw new InputMismatchException();
					res *= 10;
					res += c - '0';
					c = snext();
				} while (!isSpaceChar(c));
				return res * sgn;
			}
	 
			public long nextLong()
		    {
				int c = snext();
				while (isSpaceChar(c)) 
				{
					c = snext();
				}
				int sgn = 1;
				if (c == '-') 
				{
					sgn = -1;
					c = snext();
				}
				long res = 0;
				do 
				{
					if (c < '0' || c > '9')
						throw new InputMismatchException();
					res *= 10;
					res += c - '0';
					c = snext();
				} while (!isSpaceChar(c));
				return res * sgn;
			}
	 
			public int[] nextIntArray(int n) 
			{
				int a[] = new int[n];
				for (int i = 0; i < n; i++) 
				{
					a[i] = nextInt();
				}
				return a;
			}
	 
			public String readString()
		    {
				int c = snext();
				while (isSpaceChar(c)) 
				{
					c = snext();
				}
				StringBuilder res = new StringBuilder();
				do 
				{
					res.appendCodePoint(c);
					c = snext();
				} while (!isSpaceChar(c));
				return res.toString();
			}
	 
			public String nextLine() 
			{
				int c = snext();
				while (isSpaceChar(c))
					c = snext();
				StringBuilder res = new StringBuilder();
				do 
				{
					res.appendCodePoint(c);
					c = snext();
				} while (!isEndOfLine(c));
				return res.toString();
			}
	 
			public boolean isSpaceChar(int c) 
			{
				if (filter != null)
					return filter.isSpaceChar(c);
				return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
			}
	 
			private boolean isEndOfLine(int c) 
			{
				return c == '\n' || c == '\r' || c == -1;
			}
	 
			public interface SpaceCharFilter
		    {
				public boolean isSpaceChar(int ch);
			}
		}
		public static long c = 0;
		
		public static long mod = 1000000007;
		public static int d;
		public static int p;
		public static int q;
		
		
		
				
		public static  int gcd(int p2, int p22)
		{
		    if (p2 == 0)
		        return (int) p22;
		    return gcd(p22%p2, p2);
		}
		
			
		
		public static int findGCD(int arr[], int n)
		{
		    int result = arr[0];
		    for (int i=1; i<n; i++)
		        result = gcd(arr[i], result);
		 
		    return result;
		}
		
	
	    public static long lcm(int[] numbers) {
	        long lcm = 1;
	        int divisor = 2;
	        while (true) {
	            int cnt = 0;
	            boolean divisible = false;
	            for (int i = 0; i < numbers.length; i++) {
	                if (numbers[i] == 0) {
	                    return 0;
	                } else if (numbers[i] < 0) {
	                    numbers[i] = numbers[i] * (-1);
	                }
	                if (numbers[i] == 1) {
	                    cnt++;
	                }
	                if (numbers[i] % divisor == 0) {
	                    divisible = true;
	                    numbers[i] = numbers[i] / divisor;
	                }
	            }
	            if (divisible) {
	                lcm = lcm * divisor;
	            } else {
	                divisor++;
	            }
	            if (cnt == numbers.length) {
	                return lcm;
	            }
	        }
	    }
		public static long fact(long n) {
			
			long factorial = 1;
			 for(int i = 1; i <= n; i++)
		        {
		            factorial *= i;
		        }
			 return factorial;
		}
		
		
		
		public static int lowerLimit(int[] a, int n) {
			int ans = 0;
			
			int ll = 0;
			int rl = a.length-1;
		//	System.out.println(a[rl]+" "+n);
			if(a[0]>n)
				return 0;
			if(a[0]==n)
				return 1;
			else if(a[rl]<=n)
				return rl+1;
		
			while(ll<=rl)
			{
				
				int mid = (ll+rl)/2;
				if(a[mid]==n)
				{
					ans = mid + 1;
					break;
				}
				
				else if(a[mid]>n)
				{
					rl = mid-1;
					
				}
				else
				{
					ans = mid+1;
					ll = mid+1;
				}
			}
			
			return ans; 
		}
		
		
		public static long choose(long total, long choose){
		    if(total < choose)
		        return 0;
		    if(choose == 0 || choose == total)
		        return 1;
		    return (choose(total-1,choose-1)+choose(total-1,choose))%mod;
		}
		
		public static int[] suffle(int[] a,Random gen)
		{
			int n = a.length;
			for(int i=0;i<n;i++)
			{
				int ind = gen.nextInt(n-i)+i;
				int temp = a[ind];
				a[ind] = a[i];
				a[i] = temp;
			}
			return a;
		}
		
		public static int floorSearch(int arr[], int low, int high, int x)
		{
		    if (low > high)
		        return -1;
		 
		    if (x > arr[high])
		        return high;
		    int mid = (low+high)/2;
		 
		 
		    if (mid > 0 && arr[mid-1] < x && x < arr[mid])
		        return mid-1;
		 
		    if (x < arr[mid])
		        return floorSearch(arr, low, mid-1, x);
		 
		    return floorSearch(arr, mid+1, high, x);
		}
		
		
		public static void swap(int a, int b){
			int temp = a;
			a = b;
			b = temp;
		}
		public static ArrayList<Integer> primeFactorization(int n)
		{
			ArrayList<Integer> a =new ArrayList<Integer>();
			for(int i=2;i*i<=n;i++)
			{
				while(n%i==0)
				{
					a.add(i);
					n/=i;
				}
			}
			if(n!=1)
				a.add(n);
			return a;
		}
		
		
		public static void sieve(boolean[] isPrime,int n)
		{
			for(int i=1;i<n;i++)
				isPrime[i] = true;
			
			isPrime[0] = false;
			isPrime[1] = false;
			
			for(int i=2;i*i<n;i++)
			{
				if(isPrime[i] == true)
				{
					for(int j=(2*i);j<n;j+=i)
						isPrime[j] = false;
				}
			}
		}
		
		public static int GCD(int a,int b)
		{
			if(b==0)
				return a;
			else
				return GCD(b,a%b);
		}
		
		public static long GCD(long a,long b)
		{
			if(b==0)
				return a;
			else
				return GCD(b,a%b);
		}
		
		public static void extendedEuclid(int A,int B)
		{
			if(B==0)
			{
				d = A;
				p = 1 ;
				q = 0;
			}
			else
			{
				extendedEuclid(B, A%B);
				int temp = p;
				p = q;
				q = temp - (A/B)*q;
			}
		}
		
		public static long LCM(long a,long b)
		{
			return (a*b)/GCD(a,b);
		}
		
		public static int LCM(int a,int b)
		{
			return (a*b)/GCD(a,b);
		}
		
		public static int binaryExponentiation(int x,int n)
		{
		    int result=1;
		    while(n>0)
		    {
		        if(n % 2 ==1)
		            result=result * x;
		        x=x*x;
		        n=n/2;
		    }
		    return result;
		}
		
		
		public static int[] countDer(int n)
	    {
	        int der[] = new int[n + 1];
	     
	        der[0] = 1;
	        der[1] = 0;
	        der[2] = 1;
	     
	        for (int i = 3; i <= n; ++i)
	            der[i] = (i - 1) * (der[i - 1] + der[i - 2]);
	     
	        // Return result for n
	        return der;
	    }
		
		
		 static long binomialCoeff(int n, int k)
		    {
		    long C[][] = new long[n+1][k+1];
		    int i, j;
		     
		        // Calculate  value of Binomial Coefficient in bottom up manner
		    for (i = 0; i <= n; i++)
		    {
		        for (j = 0; j <= Math.min(i, k); j++)
		        {
		            // Base Cases
		            if (j == 0 || j == i)
		                C[i][j] = 1;
		      
		            // Calculate value using previosly stored values
		            else
		                C[i][j] = C[i-1][j-1] + C[i-1][j];
		          }
		     }
		      
		    return C[n][k];
		    }
		
		public static long binaryExponentiation(long x,long n)
		{
		    long result=1;
		    while(n>0)
		    {
		        if(n % 2 ==1)
		            result=result * x;
		        x=(x%mod * x%mod)%mod;
		        n=n/2;
		    }
		    return result;
		}
		
		public static int modularExponentiation(int x,int n,int M)
		{
		    int result=1;
		    while(n>0)
		    {
		        if(n % 2 ==1)
		            result=(result * x)%M;
		        x=(x*x)%M;
		        n=n/2;
		    }
		    return result;
		}
		
		public static long modularExponentiation(long x,long n,long M)
		{
		    long result=1;
		    while(n>0)
		    {
		        if(n % 2 ==1)
		            result=(result * x)%M;
		        x=(x*x)%M;
		        n=n/2;
		    }
		    return result;
		}
		
		public static int modInverse(int A,int M)
		{
		    return modularExponentiation(A,M-2,M);
		}
		
		public static long modInverse(long A,long M)
		{
		    return modularExponentiation(A,M-2,M);
		}
		
		public static boolean isPrime(int n)
		{
		    
		    if (n <= 1)  return false;
		    if (n <= 3)  return true;
		    
		    if (n%2 == 0 || n%3 == 0) 
		    	return false;
		 
		    for (int i=5; i*i<=n; i=i+6)
		    {
		        if (n%i == 0 || n%(i+2) == 0)
		           return false;
		    }
		    
		    return true;
		}
		
		static class pair implements Comparable<pair>
 
		{
			Integer x, y;
			pair(int x,int y)
			{
				this.x=x;
				this.y=y;
			}
			
			public int compareTo(pair o) {
				int result = x.compareTo(o.x);
				if(result==0)
					result = y.compareTo(o.y);
				
				return result;
			}  
			
			public String toString()
			{
				return x+" "+y;
			}
			
			public boolean equals(Object o)
			{
				if (o instanceof pair)
			    {
					pair p = (pair)o;
					return p.x == x && p.y == y ;
				}
				return false;
			}
			
			public int hashCode()
			{
				return new Long(x).hashCode()*31 + new Long(y).hashCode();
			}
		}
	
		
		static class triplet implements Comparable<triplet>
		{
			Integer x,y,z;
			triplet(int x,int y,int z)
			{
				this.x = x;
				this.y = y;
				this.z = z;
			}
			
			public  int compareTo(triplet o)
			{
				int result = x.compareTo(o.x);
				if(result==0)
					result =  y.compareTo(o.y);
				if(result==0)
					result = z.compareTo(o.z);

				return result;
			}
			
			public boolean equlas(Object o)
			{
				if(o instanceof triplet)
				{
					triplet p = (triplet)o;
					return x==p.x && y==p.y && z==p.z;
				}
				return false;
			}
			
			public String toString()
			{
				return x+" "+y+" "+z;
			}
			public int hashCode()
			{
				return new Long(x).hashCode()*31 + new Long(y).hashCode() + new Long(z).hashCode(); 
			}
		}
		

		
		
		
	
		static class node implements Comparable<node>
 
		{
			Integer x, y, z;
			node(int x,int y, int z)
			{
				this.x=x;
				this.y=y;
				this.z=z;
			}
			
			public int compareTo(pair o) {
				int result = x.compareTo(o.x);
				if(result==0)
					result = y.compareTo(o.y);
				if(result==0)
					result = z.compareTo(z); 
				return result;
			}
 
			@Override
			public int compareTo(node o) {
				// TODO Auto-generated method stub
				return 0;
			}  
		}
	}
	
		
	
	