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.io.*;
  public class Sol{
   
  /*   
  ->check n=1, int overflow , array bounds , all possibilites(dont stuck on 1 approach)
  ->Problem = Observation(constraints(m<=n/3 or k<=min(100,n)) 
             + Thinking + Technique (seg_tree,binary lift,rmq,bipart,dp,connected comp etc)
  ->solve or leave it (- tutorial improves you in minimal way -)
  */
   
  public static void main (String []args) {
  //precomp();
   
  int times=1;while(times-->0){solve();}out.close();}
  
  static void solve(){
   int n=ni();
   long a[]=new long[n+1];
   for(int i=1;i<=n;i++)a[i]=ni();
  
   long ans=0;
   for(int i=1;i<=n;i++){
      if(i%2==1){
        long r=a[i];
        for(int j=i+1;j<=n&&r>=0;j++){
          if(j%2==0){
             long delta=r-a[i];
             long curr=a[j];
             if(delta>a[j]){}
             else{
               if(delta>0){ans++;curr-=delta;}
               ans+=Math.min(a[i],curr);
               a[i]-=curr; 
             }
             r-=a[j];
          }
          else{r+=a[j];}//out.println(i+" "+j+" "+ans);
        }
      }
   }
   out.println(ans); 
   return;
  }
  
  //-----------------Utility--------------------------------------------
  
  static long gcd(long a,long b){if(b==0)return a; return gcd(b,a%b);}
   
  static int Max=Integer.MAX_VALUE; static long mod=1000000007;
   
  //static int v(char c){return (int)(c-'a');}
   
  public static long power(long x, long y )
      {
          //0^0 = 1
          long res = 1L;
          x = x%mod;
          while(y > 0)
          {
              if((y&1)==1)
                  res = (res*x)%mod;
              y >>= 1;
              x = (x*x)%mod;
          }
          return res;
      }
   
  static class Pair implements Comparable<Pair>{
          int id;int value;Pair next;
          public Pair(int id,int value) {
           
            this.id=id;this.value=value;next=null;
          }
          @Override
          public int compareTo(Pair p){return Long.compare(value,p.value);}
      }
  
   
  //----------------------I/O---------------------------------------------
   
  static InputStream inputStream = System.in;
  static OutputStream outputStream = System.out;
  static FastReader in=new FastReader(inputStream);
  static PrintWriter out=new PrintWriter(outputStream);
   
  static class FastReader 
  { 
    BufferedReader br; 
    StringTokenizer st; 
   
          FastReader(InputStream is) {
              br = new BufferedReader(new InputStreamReader(is));
          }
   
    public String next() 
    { 
        while (st == null || !st.hasMoreElements()) 
        { 
            try
            { 
                st = new StringTokenizer(br.readLine()); 
            } 
            catch (IOException  e) 
            { 
                e.printStackTrace(); 
            } 
        } 
        return st.nextToken(); 
    } 
   
    public int nextInt() 
    { 
        return Integer.parseInt(next()); 
    } 
   
    public long nextLong() 
    { 
        return Long.parseLong(next()); 
    } 
   
   public  double nextDouble() 
    { 
        return Double.parseDouble(next()); 
    } 
   
   
    String nextLine() 
    { 
        String str = ""; 
        try
        { 
            str = br.readLine(); 
        } 
        catch (IOException e) 
        { 
            e.printStackTrace(); 
        } 
        return str; 
    } 
  }
  /*static int ni() {
          try {
              boolean in = false;
              int res = 0;
              for (;;) {
                  int b = System.in.read() - '0';
                  if (b >= 0) {
                      in = true;
                      res = 10 * res + b;
                  }
                  else if (in)
                      return res;
              }
          } catch (IOException e) {
              throw new Error(e);
          }
      }*/
  static int ni(){return in.nextInt();}
  static long nl(){return in.nextLong();}
  static double nd(){return in.nextDouble();}
  static String ns(){return in.nextLine();}
  }