#include <cstdio>
#include <iostream>
#include <string>
#include <sstream>
#include <stack>
#include <algorithm>
#include <cmath>
#include <queue>
#include <map>
#include <set>
#include <cstdlib>
#include <bitset>
#include <tuple>
#include <assert.h>
#include <deque>
#include <bitset>

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

const long long MAX = 5100000;
const long long INF = 1LL << 60;
const long long MOD = 1'000'000'007LL;
const long long mod = 998244353LL;

using namespace std;
typedef unsigned long long ull;
typedef long long ll;


int main()
{
	ll n; scanf("%lld", &n);
	vector<ll> b(n); for (ll i = 0; i < n; i++) scanf("%lld", &b[i]);
	ll cnt = 0;
	ll r = 0;
	ll res = 0;
	for (ll i = 0; i < n; i++) {
		res += (cnt + r);
		if (b[i] > 1) {
			ll sub = min(cnt, b[i] - 1);
			cnt -= sub;
			b[i] -= sub;
			if (b[i] > 1) {
				r += b[i] - 1;
			}
		}
		else if (b[i] == 0) {
			if (r > 0) {
				r--;
			}
			else {
				cnt++;
			}
		}
	}
	cout << res << endl;
	return 0;
}