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.*;
  
  public class Codeforces
  {
      public static void main(String args[])throws Exception
      {
          BufferedReader bu=new BufferedReader(new InputStreamReader(System.in));
          StringBuilder sb=new StringBuilder();
          int n=Integer.parseInt(bu.readLine());
          int a[]=new int[n],i;
          String s[]=bu.readLine().split(" ");
          for(i=0;i<n;i++) a[i]=Integer.parseInt(s[i]);
  
          long ans=0;
          for(i=0;i<n;i++)
          if(i%2==0)
          {
              Stack<Integer> st=new Stack<>();
              int c,d,j; long cur=a[i];
              st.add(a[i]);
              for(j=i+1;j<n;j++)
              {
                  if(j%2==0) {st.add(a[j]); cur+=a[j];}
                  else
                  {
                      cur-=a[j];
                      c=a[j];
                      while(st.size()>1 && c-st.peek()>=0) c-=st.pop();
                      if(st.size()>1)
                      {
                          d=st.pop();
                          d-=c;
                          st.add(d);
                      }
                      else if(st.size()==1)
                      {
                          if(c!=a[j]) ans++;
                          d=st.pop();
                          ans+=Math.min(d,c);
                          d-=c;
                          st.add(Math.max(0,d));
                      }
                  }
                  if(cur<0) break;
              }
              //System.out.println();
          }
          System.out.println(ans);
      }
  }