#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const double PI = acos(-1);
const ll MOD = 1000000007;

int main() {
  int N; cin >> N;
  vector<int> B(N);
  for (int i = 0; i < N; i++) cin >> B[i];
  queue<int> que;
  stack<int> s;
  ll count = 0;
  for (int i = 0; i < N; i++) {
    if (B[i] == 0 && s.empty()) que.push(i);
    else if (B[i] > 1 && que.empty()) {
      for (int j = 0; j < B[i]-1; j++) s.push(i);
    }
    else if (B[i] == 0) {
      int j = s.top();
      s.pop();
      count += (i - j);
    } else {
      while (!que.empty() && B[i] > 1) {
        int j = que.front();
        que.pop();
        count += (i-j);
        B[i]--;
      }
      if (B[i] > 1) {
        for (int j = 0; j < B[i]-1; j++) s.push(i);
      }
    }
  }
  
  cout << count << endl;
}