#include <bits/stdc++.h>

#include <unistd.h>
 
using namespace std;
 
#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl
#define DEBUG_MAT(v) cerr<<#v<<endl;for(int i=0;i<v.size();i++){for(int j=0;j<v[i].size();j++) {cerr<<v[i][j]<<" ";}cerr<<endl;}
 
 
typedef long long ll;
#define int ll
 
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
template<class S, class T> pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first + t.first, s.second + t.second); }
template<class S, class T> pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first - t.first, s.second - t.second); }
template<class S, class T> ostream& operator<<(ostream& os, pair<S, T> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
#define X first
#define Y second
#define rep(i,n) for(int i=0;i<(n);i++)
#define rep1(i,n) for(int i=1;i<=(n);i++)
#define rrep(i,n) for(int i=(n)-1;i>=0;i--)
#define rrep1(i,n) for(int i=(n);i>0;i--)
#define REP(i,a,b) for(int i=a;i<b;i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(),c.end()
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a = b; return 1; } return 0; }
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = (ll)1e18 + 1;
const ll MOD = 1000000007;
//const ll MOD = 998244353;
const double pi = 3.14159265358979323846;
#define Sp(p) cout<<setprecision(15)<< fixed<<p<<endl;
int dx_ori[4] = { -1,0, 1,0 }, dy_ori[4] = { 0,1,0,-1 };
int dx[4] = {0, 0, 0, 0}, dy[4] = {0, 0, 0, 0};
int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };
#define fio() cin.tie(0); ios::sync_with_stdio(false);
// #define mp make_pair
//#define endl '\n'

/*
ll p, q, a;
ll price(ll x, ll p, ll a) {
  return x + p * x / 100 + a;
}

signed main() {
  cin >> p >> q >> a;
  ll ma = 1000000000;

  if (p <= q) {
    cout << ma << endl;
    return 0;
  }

  
  if (a == 0) {
    if (p >= q) {
      cout << 0 << endl;
    }
    else {
      int ans = 0;
      rep1 (x, ma) {
        ll y1 = price(x, p, 0), y2 = price(x, q, a);
        if (y1 <= y2) ans++;
      }
      cout << ma - ans << endl;
    }
    return 0;
  }

  ll ok = 0, ng = ma + 1;
  while (ok + 1 < ng) {
    ll mid = (ok + ng) / 2;
    if (price(mid, p, 0) < price(mid, q, a)) {
      ok = mid;
    }
    else {
      ng = mid;
    }
  }

  ok = max(0LL, ok - 10000);
  ll ans = ok;
  for (ll x = ok + 1; x <= min(ma, ok + 10000); x++) {
    if (price(x, p, 0) < price(x, q, a)) ans++;
  }
  cout << ans << endl;
  
}
//*/

signed main() {
  int n;
  cin >> n;
  vl b(n);
  rep (i, n) {
    cin >> b[i];
  }

  int now = 0, left = 0;
  ll ans = 0;
  rep (i, n) {
    while (now < i) {
      if (b[i] > 0) {
        b[i]--;
        ans += (i - now);
        now++;
      }
      else break;
    }
    if (now != i) continue;
    if (b[i] >= 1) {
      ans += left;
      left += b[i] - 1;
      now++;
    }
    else {
      if (left > 0) {
        ans += left;
        left--;
        now++;
      }
    }
  }
  cout << ans << endl;
}