#include <bits/stdc++.h>
//#include <atcoder/all>
//using namespace atcoder;
using namespace std;
const int INF = 1e9;
using ll = long long;
using ull = unsigned long long;
using inv = vector<int>;
using stv = vector<string>;
using pint = pair<int,int>;
#define FOR(i,l,r) for(int i=(l); i<(r); i++)
#define rep(i,r) for(int i=0; i<(r); i++)
#define repl(i,r) for(long long i=0; i<(r); i++)
#define FORl(i,l,r) for(long long i=(l); i<(r); i++)
#define INFL ((1LL<<62)-(1LL<<31))
#define pb(x) push_back(x)

int main(){
  int N;
  cin >> N;
  vector<ll> A(N);
  rep(i,N) cin >> A[i];

  int len = 1;

  int sentou = A[N-1];

  ll ans = 0;

  for(int i=N-1; i>=1; i--){
    if(sentou-A[i-1] == 1){
      sentou = A[i-1];
      
    }else{
      ans += sentou-A[i-1]-1;
      sentou = A[i-1];
      
    }
  }
  //cout << sentou << ans<< endl;

  if(A[0] != 1){
    ans += sentou-1;
  }
  cout << ans << endl;

}