#include <bits/stdc++.h>
#pragma GCC optimize("O3")
#pragma GCC target("tune=native")
using namespace std;
const int MAX = 1e6 + 5;
const long long MOD = 1000000007;
const long long MOD2 = 2010405347;
const long long INF = 2e18;
const int dr[] = {1, 0, -1, 0, 1, 1, -1, -1, 0};
const int dc[] = {0, 1, 0, -1, 1, -1, 1, -1, 0};
const double pi = acos(-1);
const double EPS = 1e-9;
const int block = 315;
inline long long pw(long long x, long long y, long long md = MOD) {
  long long ret = 1;
  while (y) {
    if (y & 1) ret = ret * x % md;
    x = x * x % md, y >>= 1;
  }
  return ret;
}
int n, x[MAX], y[MAX], z[MAX], nw;
long long ans;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n;
  for (int i = (1); i <= (n); ++i) {
    cin >> x[i];
    y[i] = max(y[i - 1], x[i] + 1);
  }
  for (int i = (n); i >= (1); --i) {
    nw = max(nw - 1, x[i] + 1);
    z[i] = nw;
  }
  for (int i = (1); i <= (n); ++i) {
    nw = max(y[i], z[i]);
    ans += nw - x[i] - 1;
  }
  cout << ans << '\n';
  return 0;
}
