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.*;
  import java.math.*;
  
  public class C {
  	public static void main(String[] args) throws IOException {
  
  		/**/
  		Scanner sc = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
  		/*/
  		Scanner sc = new Scanner(new BufferedReader(new InputStreamReader(new FileInputStream("src/c.in"))));
  		/**/
  		
  		int n = sc.nextInt();
  		int[] c = new int[n];
  		for (int i = 0; i < n; ++i) {
  			c[i] = sc.nextInt();
  		}
  		if (n%2==1)
  			c = Arrays.copyOfRange(c, 0, n-1);
  		n = n/2*2;
  		long ans = 0;
  		for (int i = 0; i < n; i+=2) {
  			long min = 1;
  			long max = c[i];
  			for (int j = i+1; j < n; j++) {
  				long num = c[j];
  				if (j%2==1) {
  					long nm = Math.min(min-1, num);
  					max -= nm;
  					min -= nm;
  					num -= nm;
  					if (num > 0) {
  						min = 0;
  						long nmx = Math.min(num,max);
  						ans += nmx;
  						num -= nmx;
  						max -= nmx;
  						if (num > 0)
  							break;
  					}
  				} else {
  					min += num;
  					max += num;
  				}
  			}
  		}
  		System.out.println(ans);
  	}
  }