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 CF1556C extends PrintWriter {
  	CF1556C() { super(System.out, true); }
  	Scanner sc = new Scanner(System.in);
  	public static void main(String[] $) {
  		CF1556C o = new CF1556C(); o.main(); o.flush();
  	}
  
  	void main() {
  		int n = sc.nextInt();
  		if (n % 2 == 1)
  			n--;
  		n /= 2;
  		int[] aa = new int[n];
  		long ans = 0;
  		for (int i = 0; i < n; i++) {
  			int a = sc.nextInt();
  			int b = sc.nextInt();
  			ans += Math.min(a, b);
  			aa[i] = a - b;
  		}
  		for (int i = 0; i < n; i++) {
  			long a = aa[i];
  			if (a >= 0) {
  				long b = 0;
  				for (int j = i + 1; j < n; j++)
  					if ((b += aa[j]) <= 0) {
  						ans += Math.min(a, -b) + 1;
  						if ((a += b) < 0)
  							break;
  						b = 0;
  					}
  			}
  		}
  		println(ans);
  	}
  }
  