#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios::sync_with_stdio(false); cin.tie(0);
  int n;
  cin >> n;
  vector<int> b(n);
  for (int i = 0; i < n; i++) {
    cin >> b[i];
  }
  int l = 0, r = 0;
  long long ans = 0;
  for (int i = 0; i < n; i++) {
    if (b[i] > 1) {
      b[i]--;
    } else {
      while (b[l] == 0 && l < i) l++;
      if (b[l] > 0) {
        ans += i - l;
        b[l]--;
        continue;
      }
      while (b[r] == 0 && r < n) r++;
      if (r < n && b[r] > 0) {
        ans += r - i;
        b[r]--;
      }
    }
  }
  cout << ans << '\n';
  return 0;
}