#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, one = 0;
  int d = 0;
  for (int i = 0; i < n; i++) {
    if (b[i] == 0) {
     if (p > 0) p--, res += i - d;
     else c++;
    }else if (b[i] == 1) {
      one++;
    } else {
      if (c > 0) {
        res += c*(c+1)/2 + one;
        b[i] -= c;
        c = 0;
        one = 0;
      }
      p = b[i] - 1;
      b[i] = 1;
      if (p > 0) d = i;
    }
  }
  cout << res << endl;
  return 0;
}