#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>
#include <iomanip>
#include <limits>
#include <chrono>
#include <random>
#include <array>
#include <unordered_map>
#include <functional>
#include <complex>

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 = 1000000007LL;
//const long long mod = 998244353LL;

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

int main()
{
	/*
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	*/

	ll N; scanf("%lld", &N);
	vector<ll> b(N); for (ll i = 0; i < N; i++) scanf("%lld", &b[i]);
	priority_queue<pair<ll, ll>, vector<pair<ll, ll>>, greater<>> am;
	priority_queue<ll, vector<ll>, greater<>> lc;
	ll res = 0;
	for (ll i = 0; i < N; i++) {
		if (b[i] > 1) {
			ll tmp = b[i] - 1;
			if (lc.empty()) {
				am.emplace(i, tmp);
			}
			else {
				while (!lc.empty() && tmp > 0) {
					res += i - lc.top();
					lc.pop();
					tmp--;
				}
				if (tmp > 0) {
					am.emplace(i, tmp);
				}
			}
		}
		else if (b[i] == 0) {
			if (am.empty()) lc.push(i);
			else {
				ll id, tmp;
				tie(id, tmp) = am.top();
				am.pop();
				res += i - id;
				tmp--;
				if (tmp > 0) am.emplace(id, tmp);
			}
		}
	}
	cout << res << endl;
	return 0;
}