#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < (n);i++)
#define sz(x) int(x.size())
typedef long long ll;
typedef pair<int,int> P;

int main(){
  int n;
  cin >> n;
  vector<int> b(n);
  rep(i,n) cin >> b[i];
  ll res = 0;
  ll c = 0, p = 0;
  for (int i = 0; i < n; i++) {
    if (b[i] == 0) {
      if (p) p--, b[i]++;
      else c++;
    } else if (b[i] > 1) {
      while (c && (b[i] > 1)) c--, b[i]--;
      if (b[i] > 1) p += b[i]-1;
      b[i] = 1;
    }
    res += c;
    res += p;
  }
  cout << res << endl;
  return 0;
}