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.math.*;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

     //--------------->>>>IF YOU ARE HERE FOR QUICKSORT HACK THEN SORRY NO HACK FOR YOU<<<-------------------

public class a{ 
     static long[] count,count1,count2;
     static boolean[] prime;
     static Node[] nodes,nodes1,nodes2;
     static NodeInt[] nodesInt,nodesInt1;
     static long[] arr;
     static long[][] cost;
     static int[] arrInt,darrInt,farrInt;
     static long[][] dp;
     static char[] ch,ch1;
     static long[] darr,farr;
     static long[][] mat,mat1;
     static boolean[] vis;
     static long x,h;
     static long maxl;
     static double dec;
     static long mx = (long)1e7;
     static long inf = (long)1e15;
     static String s,s1,s2,s3,s4;
     static long minl;   
     static long mod = ((long)(1e9))+7;
     // static int minl = -1;
     // static long n;
     static int n,n1,n2,q,r1,c1,r2,c2;
     static long a;
     static long b;
     static long c;
     static long d;
     static long y,z;
     static int m;
     static int ans;
     static long k;
     static FastScanner sc;
     static String[] str,str1;
     static Set<Integer> set,set1,set2;
     static SortedSet<Long> ss;
     static List<Long> list,list1,list2,list3;
     static PriorityQueue<Node> pq,pq1;
     static LinkedList<Node> ll,ll1,ll2;
     static Map<Integer,List<Integer>> map1;
     static Map<Long,Integer> map;
     static StringBuilder sb,sb1,sb2;
     static int index;
     static long[] sum;
     static int[] dx = {0,-1,0,1,-1,1,-1,1};
     static int[] dy = {-1,0,1,0,-1,-1,1,1};

     // public static void solve(){

     //    FastScanner sc = new FastScanner();
     //    // int t = sc.nextInt();
     //    int t = 1;
     //    for(int tt = 0 ; tt < t ; tt++){

     //        // s = sc.next();
     //        // s1 = sc.next();

     //        n = sc.nextInt();

     //        m = sc.nextInt();

     //        // int k = sc.nextInt();

     //        // sb = new StringBuilder();

     //        map = new HashMap<>();
     //        map1 = new HashMap<>();

     //        // q = sc.nextInt();
     //        // sb = new StringBuilder();
     //        // long k = sc.nextLong();

     //        // ch = sc.next().toCharArray();

     //        // count = new int[200002];
           
     //        // m = sc.nextInt();

     //        for(int o = 0 ; o < m ; o++){

     //            int l = sc.nextInt()-1;
     //            int r = sc.nextInt()-1;

     //        }

     //    }
        
     // }

     //--------------->>>>IF YOU ARE HERE FOR QUICKSORT HACK THEN SORRY NO HACK FOR YOU<<<-------------------

     public static void solve(){
        
        long v1 = sc.nextInt();
        long v2 = sc.nextInt();
        long t = sc.nextInt();
        long d = sc.nextInt();

        long ans = v1 + v2;
        long[] count = new long[(int)t-1];
        for(long i = 1 ; i <= (t-2)  ; i++){
            v1 += d;
            count[(int)i] = v1;
        }

        for(long i = (t-2) ; i > 0 ; i--){
            v2 += d;
            count[(int)i] = Math.min(v2,count[(int)i]);
            ans += count[(int)i];
        }

        System.out.println(ans);

     }
        
     public static void main(String[] args) {

            sc = new FastScanner();
            // Scanner sc = new Scanner(System.in);
            // int t = sc.nextInt();
            int t = 1;
            // int l = 1;
            while(t > 0){
                
                // n = sc.nextInt();
                // n = sc.nextLong();
                // k = sc.nextLong();
                // x = sc.nextLong();
                // y = sc.nextLong();
                // z = sc.nextLong();

                // a = sc.nextLong();
                // b = sc.nextLong();
                // c = sc.nextLong();
                // d = sc.nextLong();

                // x = sc.nextLong();
                // y = sc.nextLong();
                // z = sc.nextLong();
                // d = sc.nextLong();

                // n = sc.nextInt();

                // n1 = sc.nextInt();

                // m = sc.nextInt();
                // q = sc.nextInt();

                // k = sc.nextLong();

                // x = sc.nextLong();
                // d = sc.nextLong();

                // s = sc.next();

                // ch = sc.next().toCharArray();
                // ch1 = sc.next().toCharArray();

                // n = 6;

                // arr = new long[n];
                // for(int i = 0 ; i < n ; i++){
                //     arr[i] = sc.nextLong();
                // }

                // arrInt = new int[n];
                // for(int i = 0 ; i < n ; i++){
                //     arrInt[i] = sc.nextInt();
                // }
                // x = sc.nextLong();
                // y = sc.nextLong();
                // ch = sc.next().toCharArray();
                // m = n;
                // m = sc.nextInt();
                // darr = new long[m];
                // for(int i = 0 ; i < m ; i++){
                //     darr[i] = sc.nextLong();
                // }
                // k = sc.nextLong();
                // m = n;
                // darrInt = new int[n];
                // for(int i = 0 ; i < n ; i++){
                //     darrInt[i] = sc.nextInt();
                // }

                // farrInt = new int[m];
                // for(int i = 0; i < m ; i++){
                //     farrInt[i] = sc.nextInt();
                // }

                // m = n;
                // mat = new long[n][m];
                // for(int i = 0 ; i < n ; i++){
                //     for(int j = 0 ; j < m ; j++){
                //         mat[i][j] = sc.nextLong();
                //     }
                // }

                // m = n;
                // mat = new char[n][m];
                // for(int i = 0 ; i < n ; i++){
                //     String s = sc.next();
                //     for(int j = 0 ; j < m ; j++){
                //         mat[i][j] = s.charAt(j);
                //     }
                // }

                // str = new String[n];
                // for(int i = 0 ; i < n ; i++)
                //     str[i] = sc.next();

                // nodes = new Node[n];
                // for(int i = 0 ; i < n ;i++)
                //     nodes[i] = new Node(sc.nextLong(),sc.nextLong());

                // nodesInt = new NodeInt[n];
                // for(int i = 0 ; i < n ;i++)
                //     nodesInt[i] = new NodeInt(sc.nextInt(),sc.nextInt(),i);

                 solve();       
                 t -= 1;
            }

     }

    public static int log(long n,long base){

         if(n == 0 || n == 1)
             return 0;

         if(n == base)
             return 1;

         double num = Math.log(n);
         double den = Math.log(base);

         if(den == 0)
             return 0;

         return (int)(num/den);
     }

    public static boolean isPrime(long n) { 
    // Corner cases 
        if (n <= 1)  
            return false; 

        if (n <= 3)  
            return true; 
      
        // This is checked so that we can skip  
        // middle five numbers in below loop 
        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; 
    } 

    public static long gcd(long a, long b)
    {
      if (b == 0)
        return a;
      return gcd(b, a % b); 
    }

    public static long lcm(long a, long b)
    {
      return (b/gcd(b, a % b)) * a; 
    }

     public static long mod_inverse(long a,long mod){
        long x1=1,x2=0;
        long p=mod,q,t;
        while(a%p!=0){
          q = a/p;
          t = x1-q*x2;
          x1=x2; x2=t;
          t=a%p;
          a=p; p=t;
        }
        return x2<0 ? x2+mod : x2;
    }

     public static void swap(long[] curr,int i,int j){
         long temp = curr[j];
         curr[j] = curr[i];
         curr[i] = temp;
     }

    static final Random random=new Random();

    static void ruffleSortLong(long[] a) {
        int n=a.length;//shuffle, then sort 
        for (int i=0; i<n; i++) {
            int oi=random.nextInt(n);
            long temp=a[oi];
            a[oi]=a[i]; a[i]=temp;
        }
        Arrays.sort(a);
    }

    static void ruffleSortInt(int[] a) {
        int n=a.length;//shuffle, then sort 
        for (int i=0; i<n; i++) {
            int oi=random.nextInt(n);
            int temp=a[oi];
            a[oi]=a[i]; a[i]=temp;
        }
        Arrays.sort(a);
    }
    
    static void ruffleSortChar(char[] a) {
        int n=a.length;//shuffle, then sort 
        for (int i=0; i<n; i++) {
            int oi=random.nextInt(n);
            char temp=a[oi];
            a[oi]=a[i]; a[i]=temp;
        }
        Arrays.sort(a);
    }

    static long binomialCoeff(long n, long k){ 

        long res = 1; 
  
        // Since C(n, k) = C(n, n-k) 
        if (k > n - k) 
            k = n - k; 
  
        // Calculate value of 
        // [n * (n-1) *---* (n-k+1)] / [k * (k-1) *----* 1] 
        for (long i = 0; i < k; ++i) { 
            res = (res*(n - i)); 
            res = (res/(i + 1)); 
        } 

        return res; 
    } 

    static long[] fact;

    public static long inv(long n){
        return power(n, mod-2);
    }

    public static void fact(int n){
        fact = new long[n+1];
        fact[0] = 1;
        for(int j = 1;j<=n;j++)
            fact[j] = (fact[j-1]*(long)j)%mod;               
    }

    public static long binom(int n, int k){
        fact(n+1);
        long prod = fact[n];
        prod*=inv(fact[n-k]);
        prod%=mod;
        prod*=inv(fact[k]);
        prod%=mod;
        return prod;    
    }

    static long power(long x, long y){
        if (y == 0)
            return 1;
        if (y%2 == 1)
            return (x*power(x, y-1))%mod;
        return power((x*x)%mod, y/2)%mod;
    }

    static void sieve(int n){ 

        prime = new boolean[n+1]; 
        for(int i=2;i<n;i++) 
            prime[i] = true; 
          
        for(int p = 2; p*p <=n; p++) 
        { 
            if(prime[p] == true) 
            { 
                for(int i = p*p; i <= n; i += p) 
                    prime[i] = false; 
            } 
        } 

    }

    static class Node{
        long first;
        long second;
        Node(long f,long s){
            this.first = f;
            this.second = s;
        }
    }

    static class NodeInt{
        int first;
        int second;
        int third;
        NodeInt(int f,int s,int third){
            this.first = f;
            this.second = s;
            this.third = third;
        }
    }

    static long sq(long num){
        return num*num;
    }

     static class FastScanner {

                BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
                StringTokenizer st=new StringTokenizer("");
                String next() {
                        while (!st.hasMoreTokens())
                                try {
                                        st=new StringTokenizer(br.readLine());
                                } catch (IOException e) {
                                        e.printStackTrace();
                                }
                        return st.nextToken();
                }
                
                int nextInt() {
                        return Integer.parseInt(next());
                }
                int[] readArray(int n) {
                        int[] a=new int[n];
                        for (int i=0; i<n; i++) a[i]=nextInt();
                        return a;
                }
                long nextLong() {
                        return Long.parseLong(next());
                }
        }

}