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.*;
  import java.text.*;
  
  public class CF_1556_C{
      //SOLUTION BEGIN
      void pre() throws Exception{}
      void solve(int TC) throws Exception{
          int N = ni();
          long[] C = new long[N];
          for(int i = 0; i< N; i++)C[i] = nl();
          long ans = 0;
          for(int i = 0; i+1< N; i+= 2){
              long neg = 0, sum = 0;
              ans += Math.min(C[i], C[i+1]);
              for(int j = i+3; j< N; j += 2){
                  sum -= C[j-2];
                  neg = Math.min(neg, sum);
                  sum += C[j-1];
                  if(-neg > C[i])break;
                  long lo = -neg, hi = C[i];
                  long nlo = lo+sum, nhi = hi+sum;
                  ans += Math.max(0, Math.min(nhi, C[j])-Math.max(0, nlo)+1);
  //                ans += Math.max(0, Math.min(C[i], -neg))
  //                ans += Math.max(0, Math.min(C[i], C[j])+neg);
              }
          }
          pn(ans);
      }
      //SOLUTION END
      void hold(boolean b)throws Exception{if(!b)throw new Exception("Hold right there, Sparky!");}
      void exit(boolean b){if(!b)System.exit(0);}
      static void dbg(Object... o){System.err.println(Arrays.deepToString(o));}
      final long IINF = (long)1e17;
      final int INF = (int)1e9+2;
      DecimalFormat df = new DecimalFormat("0.00000000000");
      double PI = 3.141592653589793238462643383279502884197169399, eps = 1e-8;
      static boolean multipleTC = false, memory = true, fileIO = false;
      FastReader in;PrintWriter out;
      void run() throws Exception{
          long ct = System.currentTimeMillis();
          if (fileIO) {
              in = new FastReader("");
              out = new PrintWriter("");
          } else {
              in = new FastReader();
              out = new PrintWriter(System.out);
          }
          //Solution Credits: Taranpreet Singh
          int T = multipleTC? ni():1;
          pre();
          for (int t = 1; t <= T; t++) solve(t);
          out.flush();
          out.close();
          System.err.println(System.currentTimeMillis() - ct);
      }
      public static void main(String[] args) throws Exception{
          if(memory)new Thread(null, new Runnable() {public void run(){try{new CF_1556_C().run();}catch(Exception e){e.printStackTrace();System.exit(1);}}}, "1", 1 << 28).start();
          else new CF_1556_C().run();
      }
      int[][] make(int n, int e, int[] from, int[] to, boolean f){
          int[][] g = new int[n][];int[]cnt = new int[n];
          for(int i = 0; i< e; i++){
              cnt[from[i]]++;
              if(f)cnt[to[i]]++;
          }
          for(int i = 0; i< n; i++)g[i] = new int[cnt[i]];
          for(int i = 0; i< e; i++){
              g[from[i]][--cnt[from[i]]] = to[i];
              if(f)g[to[i]][--cnt[to[i]]] = from[i];
          }
          return g;
      }
      int[][][] makeS(int n, int e, int[] from, int[] to, boolean f){
          int[][][] g = new int[n][][];int[]cnt = new int[n];
          for(int i = 0; i< e; i++){
              cnt[from[i]]++;
              if(f)cnt[to[i]]++;
          }
          for(int i = 0; i< n; i++)g[i] = new int[cnt[i]][];
          for(int i = 0; i< e; i++){
              g[from[i]][--cnt[from[i]]] = new int[]{to[i], i, 0};
              if(f)g[to[i]][--cnt[to[i]]] = new int[]{from[i], i, 1};
          }
          return g;
      }
      int find(int[] set, int u){return set[u] = (set[u] == u?u:find(set, set[u]));}
      int digit(long s){int ans = 0;while(s>0){s/=10;ans++;}return ans;}
      long gcd(long a, long b){return (b==0)?a:gcd(b,a%b);}
      int gcd(int a, int b){return (b==0)?a:gcd(b,a%b);}
      int bit(long n){return (n==0)?0:(1+bit(n&(n-1)));}
      void p(Object... o){for(Object oo:o)out.print(oo+" ");}
      void pn(Object... o){for(int i = 0; i< o.length; i++)out.print(o[i]+(i+1 < o.length?" ":"\n"));}
      void pni(Object... o){for(Object oo:o)out.print(oo+" ");out.println();out.flush();}
      String n()throws Exception{return in.next();}
      String nln()throws Exception{return in.nextLine();}
      int ni()throws Exception{return Integer.parseInt(in.next());}
      long nl()throws Exception{return Long.parseLong(in.next());}
      double nd()throws Exception{return Double.parseDouble(in.next());}
  
      class FastReader{
          BufferedReader br;
          StringTokenizer st;
          public FastReader(){
              br = new BufferedReader(new InputStreamReader(System.in));
          }
  
          public FastReader(String s) throws Exception{
              br = new BufferedReader(new FileReader(s));
          }
  
          String next() throws Exception{
              while (st == null || !st.hasMoreElements()){
                  try{
                      st = new StringTokenizer(br.readLine());
                  }catch (IOException  e){
                      throw new Exception(e.toString());
                  }
              }
              return st.nextToken();
          }
  
          String nextLine() throws Exception{
              String str;
              try{
                  str = br.readLine();
              }catch (IOException e){
                  throw new Exception(e.toString());
              }
              return str;
          }
      }
  }