description: <image>
  
  William has a favorite bracket sequence. Since his favorite sequence is quite big he provided it to you as a sequence of positive integers c_1, c_2, ..., c_n where c_i is the number of consecutive brackets "(" if i is an odd number or the number of consecutive brackets ")" if i is an even number.
  
  For example for a bracket sequence "((())()))" a corresponding sequence of numbers is [3, 2, 1, 3].
  
  You need to find the total number of continuous subsequences (subsegments) [l, r] (l ≤ r) of the original bracket sequence, which are regular bracket sequences.
  
  A bracket sequence is called regular if it is possible to obtain correct arithmetic expression by inserting characters "+" and "1" into this sequence. For example, sequences "(())()", "()" and "(()(()))" are regular, while ")(", "(()" and "(()))(" are not.
  
  Input
  
  The first line contains a single integer n (1 ≤ n ≤ 1000), the size of the compressed sequence.
  
  The second line contains a sequence of integers c_1, c_2, ..., c_n (1 ≤ c_i ≤ 10^9), the compressed sequence.
  
  Output
  
  Output a single integer — the total number of subsegments of the original bracket sequence, which are regular bracket sequences.
  
  It can be proved that the answer fits in the signed 64-bit integer data type.
  
  Examples
  
  Input
  
  
  5
  4 1 2 3 1
  
  
  Output
  
  
  5
  
  
  Input
  
  
  6
  1 3 2 1 2 4
  
  
  Output
  
  
  6
  
  
  Input
  
  
  6
  1 1 1 1 2 2
  
  
  Output
  
  
  7
  
  Note
  
  In the first example a sequence (((()(()))( is described. This bracket sequence contains 5 subsegments which form regular bracket sequences:
  
    1. Subsequence from the 3rd to 10th character: (()(()))
    2. Subsequence from the 4th to 5th character: ()
    3. Subsequence from the 4th to 9th character: ()(())
    4. Subsequence from the 6th to 9th character: (())
    5. Subsequence from the 7th to 8th character: ()
  
  
  
  In the second example a sequence ()))(()(()))) is described.
  
  In the third example a sequence ()()(()) is described.
solution: import java.io.*;
  import java.util.ArrayList;
  import java.util.Arrays;
  import java.util.StringTokenizer;
  import java.util.List;
   import java.util.*;
   public class realfast implements Runnable 
    {
      private static final int INF = (int) 1e9;
      long in= 1000000007;
      long fac[]= new long[1000001];
      long inv[]=new long[1000001];
      public  void solve() throws IOException 
      {
         
  
  
  
  
        int n =  readInt();
        int arr[]=new int[n+1];
  
        for(int i =0;i<n;i++)
          arr[i]= readInt();
         long ans=0;
        for(int i =0;i<n;i=i+2)
        {
          long sum = 1;
          long extra =arr[i]-1;
           for(int j=i+1;j<n;j++)
           {
              if(j%2==1)
              {
                  if(arr[j]>=sum)
                  {
                      ans++;
                      ans= ans+Math.min(extra,arr[j]-sum);
                  }
  
                  sum= sum-arr[j];
                  if(sum<0)
                  {
                      if(Math.abs(sum)>extra)
                      {
                          break;
                      }
                      extra = extra - Math.abs(sum);
                      sum=0;
  
                  }
  
              }
              else
              {
                  sum= sum+arr[j];
              }
  
           }
        }
  
        out.println(ans);
  
  
         
  
      }
      
  
     
     
      public long query(long seg[] , int left, int right , int index, int l , int r)
      {   
           long inf = 100000000;
           inf = inf*inf;
          if(left>=l&&right<=r)
          {
              return seg[index];
          }
          if(l>right||left>r)
              return inf;
          int mid = left+(right-left)/2;
          return Math.min(query(seg,left,mid,2*index+1,l,r),query(seg,mid+1,right,2*index+2,l,r));
  
      }
      
      public int value (int seg[], int left , int right ,int index, int l, int r)
      {
              
              if(left>right)
              {
                return -100000000;
              }
              if(right<l||left>r)
                  return -100000000;
              if(left>=l&&right<=r)
                  return seg[index];
              int mid = left+(right-left)/2;
              int val = value(seg,left,mid,2*index+1,l,r);
              int val2 = value(seg,mid+1,right,2*index+2,l,r);
              return Math.max(val,val2);
  
      }
     
      public int gcd(int a , int b )
      {
        if(a<b)
        {
          int t =a;
          a=b;
          b=t;
        }
        if(a%b==0)
          return b ;
        return gcd(b,a%b);
      }
      public long pow(long n , long p,long m)
      {
           if(p==0)
              return 1;
          long val = pow(n,p/2,m);;
          val= (val*val)%m;
          if(p%2==0)
              return val;
          else
              return (val*n)%m;
      }
      
      
      ///////////////////////////////////////////////////////////////////////////////////////////////////////////////////
      public static void main(String[] args) {
          new Thread(null, new realfast(), "", 128 * (1L << 20)).start();
      }
   
      private static final boolean ONLINE_JUDGE = System.getProperty("ONLINE_JUDGE") != null;
      private BufferedReader reader;
      private StringTokenizer tokenizer;
      private PrintWriter out;
   
      @Override
      public void run() {
          try {
              if (ONLINE_JUDGE || !new File("input.txt").exists()) {
                  reader = new BufferedReader(new InputStreamReader(System.in));
                  out = new PrintWriter(System.out);
              } else {
                  reader = new BufferedReader(new FileReader("input.txt"));
                  out = new PrintWriter("output.txt");
              }
              solve();
          } catch (IOException e) {
              throw new RuntimeException(e);
          } finally {
              try {
                  reader.close();
              } catch (IOException e) {
                  // nothing
              }
              out.close();
          }
      }
   
      private String readString() throws IOException {
          while (tokenizer == null || !tokenizer.hasMoreTokens()) {
              tokenizer = new StringTokenizer(reader.readLine());
          }
          return tokenizer.nextToken();
      }
   
      @SuppressWarnings("unused")
      private int readInt() throws IOException {
          return Integer.parseInt(readString());
      }
   
      @SuppressWarnings("unused")
      private long readLong() throws IOException {
          return Long.parseLong(readString());
      }
   
      @SuppressWarnings("unused")
      private double readDouble() throws IOException {
          return Double.parseDouble(readString());
      }
  }
  class edge implements Comparable<edge>{
      int u ;
      int v;
      
      edge(int  u, int v)
      {
         this.u=u;
         this.v=v;
      }
      public int compareTo(edge e)
      {
          return this.v-e.v;
      }
  }