#include <iostream>
#include <vector>
#include <set>
#include <queue>
#include <stack>
#include <math.h>
#include <cassert>
#include <bitset>
#include <map>
#include <algorithm>
#include <iterator>
#include <string>
#include <utility>
#include <numeric>
#include <regex>
#include <complex>
#include <random>
#include <atcoder/all>
using namespace atcoder;
using namespace std;
using ll=long long;
using ull=unsigned long long;
using P=pair<ll,ll>;
template<typename T>using minque=priority_queue<T,vector<T>,greater<T>>;
template<typename T>bool chmax(T &a,const T &b){return (a<b?(a=b,true):false);}
template<typename T>bool chmin(T &a,const T &b){return (a>b?(a=b,true):false);}
template<typename T1,typename T2>ostream &operator<<(ostream &os,const pair<T1,T2>&p){os<<p.first<<' '<<p.second;return os;}
template<typename T1,typename T2>istream &operator>>(istream &is,pair<T1,T2>&p){is>>p.first>>p.second;return is;}
template<typename T> ostream &operator<<(ostream &os,const vector<T> &a){cout<<'{';for(int i=0;i<a.size();i++){cout<<a[i];if(i+1!=a.size())cout<<',';};cout<<'}';return os;}
template<typename T>istream &operator>>(istream &is,vector<T> &a){for(auto &i:a)is>>i;return is;}
#define reps(i,a,n) for(int i=(a);i<(n);i++)
#define rep(i,n) reps(i,0,n)
#define all(x) x.begin(),x.end()
ll myceil(ll a,ll b){return (a+b-1)/b;}
template<typename T1,typename ...T2>
void debug(const T1& x,const T2& ...y){
  #ifdef LOCAL
  cout<<x;
  ((cout<<' '<<y),...);
  cout<<'\n';
  #endif
}
void SOLVE();
int main(){
  #ifdef LOCAL
  clock_t start=clock();
  #endif
  int testcase=1;
  //cin>>testcase;
  for(int i=0;i<testcase;i++){
    SOLVE();
  }
  #ifdef LOCAL
  cout<<"time:";
  cout<<(clock()-start)/1000;
  cout<<"ms\n";
  #endif
}
void SOLVE(){
  int n;
  cin>>n;
  vector<int>a(n);
  cin>>a;
  set<pair<int,int>>s;
  int l=a[0],r=a[0];
  reps(i,1,n){
    if(a[i]==++r)continue;
    s.insert({l,r-1});
    l=r=a[i];
  }
  s.insert({l,a[n-1]});
  ll ans=0;
  while((*s.rbegin()).second!=n){
    auto c=s.rbegin();
    int x,y;
    tie(x,y)=*c;
    s.erase(*c);
    if(s.empty()){
      ans+=x-1;
      break;
    }
    c=s.rbegin();
    int xx,yy;
    tie(xx,yy)=*c;
    s.erase(*c);
    ans+=x-yy-1;
    yy+=y-x+1;
    s.insert({xx,yy});
  }
  cout<<ans<<endl;
}