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.*;
  
  //           THIS TEMPLATE MADE BY AKSH BANSAL.
  
  public class Solution {
      static class FastReader {
          BufferedReader br;
          StringTokenizer st;
  
          public FastReader() {
              br = new BufferedReader(new InputStreamReader(System.in));
          }
  
          String next() {
              while (st == null || !st.hasMoreElements()) {
                  try {
                      st = new StringTokenizer(br.readLine());
                  } catch (IOException e) {
                      e.printStackTrace();
                  }
              }
              return st.nextToken();
          }
  
          int nextInt() {
              return Integer.parseInt(next());
          }
  
          long nextLong() {
              return Long.parseLong(next());
          }
  
          double nextDouble() {
              return Double.parseDouble(next());
          }
  
          String nextLine() {
              String str = "";
              try {
                  str = br.readLine();
              } catch (IOException e) {
                  e.printStackTrace();
              }
              return str;
          }
      }
      private static boolean[] isPrime;
      private static void primes(){
          int num = (int)1e6; // PRIMES FROM 1 TO NUM
          isPrime = new boolean[num];
       
          for (int i = 2; i< isPrime.length; i++) {
             isPrime[i] = true;
          }
          for (int i = 2; i< Math.sqrt(num); i++) {
             if(isPrime[i] == true) {
                for(int j = (i*i); j<num; j = j+i) {
                   isPrime[j] = false;
                }
             }
          }
      }
      static void sort(int a[]){ // int -> long
          ArrayList<Integer> arr=new ArrayList<>(); // Integer -> Long
          for(int i=0;i<a.length;i++)
          arr.add(a[i]);
          Collections.sort(arr);
          for(int i=0;i<a.length;i++)
          a[i]=arr.get(i);
          
      }
      private static long gcd(long a, long b){
          if(b==0)return a;
          return gcd(b,a%b);
      }
      private static long pow(long x,long y){
          if(y==0)return 1;
          long temp = pow(x, y/2);
          if(y%2==1){
              return x*temp*temp;
          }
          else{
              return temp*temp;
          }
      }
  
      static PrintWriter out;
      static FastReader sc ;
      public static void main(String[] args) throws IOException {
          sc = new FastReader();
          out = new PrintWriter(System.out);
          // primes();
          // ________________________________
  
          // _______________________________
  
          // int n = sc.nextInt();
              int n  = sc.nextInt();
              long[] arr =new long[n];
              for(int i=0;i<n;i++){
                  arr[i] = sc.nextLong();
              }
          out.println(solver(arr, n));
          // ________________________________
          out.flush();
      }
  
      public static long solver(long[] arr, int n) {
          long res = 0;
          long cur = 0;
          long prev = 0;
          Stack<long[]> stack = new Stack<>();
          for(int i=0;i<n-1;i+=2){
              long temp = arr[i]-arr[i+1];
              if(temp>0){
                  stack.push(new long[]{prev, cur});
                  prev = 0;
                  cur = 0;
              }
              res+= prev+Math.min(arr[i], arr[i+1]);
              prev+=Math.min(arr[i], arr[i+1])>0?1:0;
  
              while(cur>0 && temp<0 && !stack.isEmpty()){
                  if(Math.abs(temp)>Math.abs(cur)){
                      long[] anc = stack.pop();
                      cur+=anc[1];
                      res+=anc[0];
                  }
                  else{
                      break;
                  }
              }
  
              if(cur>0 && temp<0){
                  res+=Math.min(Math.abs(cur), Math.abs(temp));
  
                  prev = Math.min(Math.abs(cur), Math.abs(temp))>0?1:0;
              }
              cur+=temp;
              if(cur == 0 && !stack.isEmpty()){
  
                  long[] anc = stack.pop();
                  cur+=anc[1];
                  res+=anc[0];
                  prev+= anc[0];
              }
              else if(cur<0){
                  prev = 0;
                  cur = 0;
              }
              // System.out.println(i+"__"+ res);
          }
          return res;
      }
  }