#ifdef DEBUG
#include"stdlibrary.h"
#else
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include<bits/stdc++.h>
using namespace std;
#endif
// #include<ext/pb_ds/assoc_container.hpp>
// #include<ext/pb_ds/tree_policy.hpp>
// #include<ext/pb_ds/tag_and_trait.hpp>
// using namespace __gnu_pbds;
// #include<boost/multiprecision/cpp_int.hpp>
// namespace multiprecisioninteger = boost::multiprecision;
// using cint=multiprecisioninteger::cpp_int;
using ll=long long;
using datas=pair<ll,ll>;
using ddatas=pair<long double,long double>;
using tdata=pair<ll,datas>;
using vec=vector<ll>;
using mat=vector<vec>;
using pvec=vector<datas>;
using pmat=vector<pvec>;
// using llset=tree<ll,null_type,less<ll>,rb_tree_tag,tree_order_statistics_node_update>;
#define For(i,a,b) for(i=a;i<(ll)b;++i)
#define bFor(i,b,a) for(i=b,--i;i>=(ll)a;--i)
#define rep(i,N) For(i,0,N)
#define rep1(i,N) For(i,1,N)
#define brep(i,N) bFor(i,N,0)
#define brep1(i,N) bFor(i,N,1)
#define all(v) (v).begin(),(v).end()
#define allr(v) (v).rbegin(),(v).rend()
#define vsort(v) sort(all(v))
#define vrsort(v) sort(allr(v))
#define uniq(v) vsort(v),(v).erase(unique(all(v)),(v).end())
#define endl "\n"
#define popcount __builtin_popcountll
#define eb emplace_back
#define print(x) cout<<x<<endl
#define printyes print("Yes")
#define printno print("No")
#define printYES print("YES")
#define printNO print("NO")
#define output(v) do{bool f=0;for(auto outi:v){cout<<(f?" ":"")<<outi;f=1;}cout<<endl;}while(0)
#define matoutput(v) do{for(auto outimat:v)output(outimat);}while(0)
// constexpr ll mod=1000000007;
constexpr ll mod=998244353;
constexpr ll inf=1LL<<60;
constexpr long double eps=1e-9;
const long double PI=acosl(-1);
template<class T,class E> ostream& operator<<(ostream& os,const pair<T,E>& p){return os<<"("<<p.first<<","<<p.second<<")";}
template<class T> ostream& operator<<(ostream& os,const vector<T>& v){
  os<<"{";bool f=false;
  for(auto& x:v){if(f)os<<",";os<<x;f=true;}
  os<<"}";
  return os;
}
template<class T> ostream& operator<<(ostream& os,const set<T>& v){
  os<<"{";bool f=false;
  for(auto& x:v){if(f)os<<",";os<<x;f=true;}
  os<<"}";
  return os;
}
template<class T> ostream& operator<<(ostream& os,const multiset<T>& v){
  os<<"{";bool f=false;
  for(auto& x:v){if(f)os<<",";os<<x;f=true;}
  os<<"}";
  return os;
}
template<class T,class E> ostream& operator<<(ostream& os,const map<T,E>& v){
  os<<"{";bool f=false;
  for(auto& x:v){if(f)os<<",";os<<x;f=true;}
  os<<"}";
  return os;
}
template<class T> inline bool chmax(T& a,const T b){bool x=a<b;if(x)a=b;return x;}
template<class T> inline bool chmin(T& a,const T b){bool x=a>b;if(x)a=b;return x;}
#ifdef DEBUG
void debugg(){cout<<endl;}
template<class T,class... Args>void debugg(const T& x,const Args&... args){cout<<" "<<x;debugg(args...);}
#define debug(...) cout<<__LINE__<<" ["<<#__VA_ARGS__<<"]:",debugg(__VA_ARGS__)
#else
#define debug(...) (void(0))
#endif

inline void startupcpp(void) noexcept{
  cin.tie(0)->sync_with_stdio(0);
  cout<<fixed<<setprecision(15);
}

ll modinv(ll a,const ll m=mod) noexcept{
  ll b=m,u=1,v=0,t;
  while(b){
    t=a/b;
    a-=t*b; swap(a,b);
    u-=t*v; swap(u,v);
  }
  return (u+m)%m;
}

ll moddevide(const ll a,const ll b) noexcept{return (a*modinv(b))%mod;}

vec modncrlistp,modncrlistm;

ll modncr(const ll n,const ll r) noexcept{
  if(n<r||r<0)return 0;
  ll i,size=modncrlistp.size();
  if(size<=n){
    modncrlistp.resize(n+1);
    modncrlistm.resize(n+1);
    if(!size){
      modncrlistp[0]=modncrlistm[0]=1;
      size++;
    }
    For(i,size,n+1)modncrlistp[i]=modncrlistp[i-1]*i%mod;
    modncrlistm[n]=modinv(modncrlistp[n]);
    for(i=n;i>size;--i)modncrlistm[i-1]=modncrlistm[i]*i%mod;
  }
  return modncrlistp[n]*modncrlistm[r]%mod*modncrlistm[n-r]%mod;
}

ll modpow(ll a,ll n,const ll m=mod){
  if(n<0)return 0;
  ll res=1;
  while(n>0){
    if(n&1)res=res*a%m;
    a=a*a%m;
    n>>=1;
  }
  return res;
}

constexpr ll gcd(const ll a,const ll b) noexcept{return (!b)?abs(a):(a%b==0)?abs(b):gcd(b,a%b);}
constexpr ll lcm(const ll a,const ll b) noexcept{return a/gcd(a,b)*b;}

// #include<atcoder/all>
// using mint=atcoder::modint998244353;
// void operator>>(istream& is,mint& v){long long x;is>>x;v=x;}
// ostream& operator<<(ostream& os,const mint& v){return os<<v.val();}
int judge(vector<int>& v){
  int ans=0;
  for(size_t i=1;i+1<v.size();++i){
    if(v[i-1]<v[i]&&v[i]>v[i+1]){
      int tmp=min(v[i-1],v[i+1])-1;
      swap(v[i],tmp);
      chmax(ans,judge(v)+1);
      swap(v[i],tmp);
    }
  }
  return ans;
}
int solve(vector<int> v){
  set<int> idlis;
  for(size_t i=1;i+1<v.size();++i){
    if(v[i-1]<v[i]&&v[i]>v[i+1])idlis.emplace(i);
  }
  int ans=0;
  while(!idlis.empty()){
    int id=*idlis.begin();
    idlis.erase(id);
    v[id]=min(v[id-1],v[id+1])-1;
    if(id>=2&&v[id-2]<v[id-1])idlis.emplace(id-1);
    if(id+2<(int)v.size()&&v[id+2]<v[id+1])idlis.emplace(id+1);
    ++ans;
  }
  return ans;
}
struct unionfind{
  private:
  int maxN;
  vector<int> par,treesize;
  public:unionfind(int N) :maxN(N),par(N),treesize(N,1){
    for(int i=0;i<maxN;++i)par[i]=i;
  }
  int root(int x){
    while(par[x]!=x){
      x=par[x]=par[par[x]];
    }
    return x;
  }
  bool merge(int x,int y){
    x=root(x);
    y=root(y);
    if(x==y)return false;
    if(x>y)swap(x,y);
    par[x]=y;
    treesize[y]+=treesize[x];
    return true;
  }
  bool merge(pair<int,int> v){
    return merge(v.first,v.second);
  }
  bool same(int x,int y){
    return root(x)==root(y);
  }
  bool same(pair<int,int> v){
    return same(v.first,v.second);
  }
  int size(int x){
    return treesize[root(x)];
  }
  void clear(){
    treesize.assign(maxN,1);
    for(int i=0;i<maxN;++i)par[i]=i;
  }
  vector<vector<int>> groups(){
    vector<vector<int>> res(maxN);
    for(int i=0;i<maxN;++i)res[root(i)].emplace_back(i);
    // return res;
    vector<vector<int>> res2;
    for(vector<int> x:res){
      if(x.size())res2.emplace_back(x);
    }
    return res2;
  }
};

ll solve2(vector<int> v){
  int N=v.size();
  unionfind dsu(N);
  ll ans=0;
  for(int i=2;i<N;++i){
    if(v[i]<v[i-1]&&(v[i-2]<v[i-1]||dsu.same(i-2,i-1))){
      dsu.merge(i-1,i);
      int nxt=i-dsu.size(i);
      while(nxt&&v[nxt]>v[i]-i+nxt+1){
        if(!dsu.same(nxt,nxt-1)&&v[nxt-1]>=v[nxt])break;
        dsu.merge(nxt,i);
        nxt=i-dsu.size(i);
      }
    }
    ans+=dsu.size(i)-1;
  }
  return ans;
}
int main(){
  startupcpp();
// int codeforces;cin>>codeforces;while(codeforces--){
  int N=10;
  cin>>N;
  vector<int> v(N);
  for(auto& x:v)cin>>x;
  print(solve2(v));
  // int cnt=0;
  // do{
  //   ++cnt;
  //   N=rand()%10+1;
  //   for(auto& x:v)x=rand()%100;
  //   if(cnt%100==0)debug(cnt);
  //   // print(judge(v));
  // }while(solve(v)==solve2(v));
  // debug(v);
}