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 Main {
      public static void main(String[] args) throws IOException {
          //BufferedReader f = new BufferedReader(new FileReader("uva.in"));
          BufferedReader f = new BufferedReader(new InputStreamReader(System.in));
          PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
          int n = Integer.parseInt(f.readLine());
          StringTokenizer st = new StringTokenizer(f.readLine());
          int[] a = new int[n];
          for(int i = 0; i < n; i++) {
              a[i] = Integer.parseInt(st.nextToken());
          }
          long ans = 0;
          for(int i = 0; i < n-1; i += 2) {
              long cur = a[i]-a[i+1];
              long ext = 0;
              ans += Math.min(a[i], a[i+1]);
              for(int j = i+3; j < n && cur >= 0; j += 2) {
                  long left = a[j]-a[j-1];
                  if(left < 0) {
                      ext += -left;
                  } else {
                      if(left < ext) {
                          ext -= left;
                      } else {
                          left -= ext;
                          ext = 0;
                          ans += Math.min(cur, left)+1;
                          cur -= left;
                      }
                  }
              }
          }
          out.println(ans);
          f.close();
          out.close();
      }
  }