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: //package com.company;
  
  import java.util.*;
  
  
  
  public class Main {
  
      public static void main(String[] args) {
          // write your code here
  
         Scanner  scanner = new Scanner(System.in);
  
        //. int t = scanner.nextInt();
         //int casen = 1;
  
  
  
              int n = scanner.nextInt();
              long[] arr = new long[n];
              for(int i =0; i < n;i++)
                  arr[i] = scanner.nextLong();
              long sol=0;
              for(int i =1; i < n;i++)
              {
                  long open = 0;
                  long from=0;
                  long closes=0;
                  from+=Math.min(arr[i-1],arr[i]);
                  open = arr[i-1]-arr[i];
                  int j =i+2;
                  long total = 0;
                  while(j<n&&open>=0)
                  {
                      closes +=arr[j]-arr[j-1];
  
                      if(closes>=0)
                      {
                          if(arr[j]-arr[j-1]>=0)
                          from += Math.min(open,Math.min(arr[j]-arr[j-1],closes))+1;
                          open -= closes;
                          closes=0;
                      }
  
                      j+=2;
                  }
                  sol += from;
                  i++;
              }
              System.out.println(sol);
  
  
  
  
  
  
      }
  
  
  
  
  
  
  
  
  
  
  }