#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <iomanip>
#include <cmath>
#include <map>
using namespace std;
using ll = long long;
constexpr ll MOD = 1e9 + 7;
constexpr int INF = 1 << 30;

int N, Q, K;
string S;
map<int, bool> m;

int main()
{
  cin.tie(0);
  ios_base::sync_with_stdio(false);
  int N; cin >> N;
  vector<int> A(N);

  for(int i = 0; i < N; ++i)
    cin >> A[i];
  
  ll ans = 0, cnt = 0;
  queue<int> q;
  for(int i = 0; i < N; ++i) {
    if(A[i] == 0) {
      if(cnt > 0) --cnt;
      else q.push(i);
    } else {
      while(A[i] > 1 && !q.empty()){
        ans += i - q.front();
        q.pop();
        --A[i];
      }
      cnt += A[i] - 1;
    }
    ans += cnt;
  }
  cout << ans << endl;

  return 0;
}