#include <bits/stdc++.h>
using namespace std;
#define rep(i, x, limit) for (long long i = (long long)x; i < (long long)limit; i++)
#define REP(i, x, limit) for (long long i = (long long)x; i <= (long long)limit; i++)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define el '\n'
#define spa " "
#define Yes cout << "Yes" << el
#define No cout << "No" << el
#define YES cout << "YES" << el
#define NO cout << "NO" << el
#define eps (1e-10)
#define Equals(a,b) (fabs((a) - (b)) < eps )
#define debug(x) cerr << #x << " = " << x << el
using ll = long long;
using ull = unsigned long long;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using vi = vector<int>;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using vs = vector<string>;
using vb = vector<bool>;
const double pi = 3.141592653589793238;
const int inf = 1073741823;
const ll infl = 1LL << 60;
const string ABC = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const string abc = "abcdefghijklmnopqrstuvwxyz";
const ll MOD = 998244353;
#include<atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using vm = vector<mint>;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  ll n;cin>>n;
  vl A(n);rep(i,0,n)cin>>A[i];
  stack<pll> st;
  for(auto a:A){
    if(st.empty())st.push({a,a});
    else{
      auto [t,e]=st.top();
      if(e==a-1){
        st.pop();
        st.push({t,a});
      }else{
        st.push({a,a});
      }
    }
  }
  ll ans=0;
  while(st.size()>1){
    auto [aa,bb]=st.top();st.pop();
    auto [cc,dd]=st.top();st.pop();
    ans+=aa-dd-1;
    st.push({cc,bb-(aa-dd-1)});
  }
  auto [aa,bb]=st.top();st.pop();
  ans+=aa-1;
  cout<<ans<<el;
}