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.util.*;
  import java.lang.*;
  import java.io.*;
  public class Main
  {
  	public static void main (String[] args) throws java.lang.Exception
  	{
  	    Scanner sc=new Scanner(System.in);
  	   // BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
  	    int t=1;
  	 //   t=sc.nextInt();
  	  //  int t=Integer.parseInt(br.readLine());
  	    while(--t>=0){
  	        int n=sc.nextInt();
  	         int a[]=new int[n];
  	        for(int i=0;i<n;i++)a[i]=sc.nextInt();
  	       long ans=0;
  	       for(int i=1;i<n;i=i+2){
  	           long x=a[i];
  	           long y=0;
  	           for(int j=i;j>=0;j--){
  	               if(j%2!=0)y=y+a[j];
  	               else{
  	                   y=y-a[j];
  	                   if(y>=0){
  	                       ans=ans+Math.max(x-y,0);
  	                       if(y<=x&&(i-j)>2)ans++;
  	                       x=Math.min(x,y);
  	                   }
  	                   else{
  	                       ans=ans+x;
  	                       if((i-j)>2)ans++;
  	                       break;
  	                   }
  	               }
  	               if(y<0)break;
  	           }
  	         //  System.out.println(ans);
  	       }
  	        
  	     
  	        System.out.println(ans);
  	    }
  	    
  	}
  }
  