#include <iostream>
#include <optional>
#include <vector>
#include <algorithm>
#include <cmath>

using namespace std;
#define int long long

struct fenwick_tree {
	vector<int> fwt;
	vector<int> base;
	fenwick_tree(vector<int>& input) {
		base = input;
		vector<int> prefixSum(input.size()+1);
		prefixSum[0] = 0;
		for (size_t i = 1; i < input.size()+1; i++)
		{
			prefixSum[i] = prefixSum[i - 1] + input[i-1];
		}
		fwt.push_back(prefixSum[0]);
		for (size_t i = 1; i < input.size()+1; i++)
		{
			int range = i - (i & (i - 1));
			fwt.push_back(prefixSum[i] - prefixSum[i - range]);
		}
	}
	int sumRange(int beg, int end) {
		end++;
		return sumFromBeginning(end) - sumFromBeginning(beg);
	}
	int sumFromBeginning(int end) {
		int sum = 0;
		while (end != 0) {
			sum += fwt[end];
			end = (end & (end - 1));
		}
		return sum;
	}
	void update(int idx, int newVal) {
		int dif = newVal - base[idx];
		base[idx] = newVal;
		idx++;
		while (idx < fwt.size()) {
			fwt[idx] += dif;
			int range = idx - (idx & (idx - 1));
			idx = (idx & (idx - 1)) + 2*range;
		}
	}
};

struct tNode {
	int val;
	int end;
	int beg;
	int par;
	tNode(int Val, int Beg, int End) {
		val = Val;
		beg = Beg;
		end = End;
		par = -1;
	}
	tNode() {};
};
struct segment_tree {
	vector<tNode> smt;
	int fullSize = 1;
	segment_tree(const vector<int>& inp) {
		for (size_t i = 0; i < 32; i++)
		{
			if (inp.size() > fullSize) fullSize *= 2;
			else break;
		}
		smt.resize(2 * fullSize - 1);
		for (size_t i = 0; i < inp.size(); i++)
		{
			smt[i] = tNode(inp[i],i,i);
		}
		int cur = 0;
		for (size_t i = fullSize; i < smt.size(); i++)
		{
			smt[i] = tNode(smt[cur].val + smt[cur + 1].val, smt[cur].beg, smt[cur + 1].end);
			smt[cur].par = i;
			smt[cur+1].par = i;
			cur += 2;
		}
	}
	int sumRange(int beg, int end) {
		int ans = 0;
		int cur = beg;
		while (true)
		{
			if (smt[cur].end==end) {
				ans += smt[cur].val;
				break;
			}
			tNode par = smt[smt[cur].par];
			if (par.beg >= cur && par.end <= end) {
				cur = smt[cur].par;
			}
			else {
				ans += smt[cur].val;
				cur = smt[cur].end + 1;
			}
		}
		return ans;
	}
	void update(int index, int value) {
		int cur = index;
		int dif = value - smt[cur].val;
		while (cur!=-1)
		{
			smt[cur].val += dif;
			cur = smt[cur].par;
		}
	}
};

signed main()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);

	/*int n, q;
	cin >> n >> q;
	vector<int> inp(n);
	for (size_t i = 0; i < n; i++)
	{
		cin >> inp[i];
	}
	segment_tree sm = segment_tree(inp);
	for (size_t i = 0; i < q; i++)
	{
		int a, b, c;
		cin >> a >> b >> c;
		if (a == 1) {
			sm.update(b, c);
		}
		else {
			c--;
			cout << sm.sumRange(b, c) << "\n";
		}
	}*/
	int n;
	cin >> n;
	vector<int> inp(n, 1);
	vector<pair<int, int>> sVec(n);
	for (size_t i = 0; i < n; i++)
	{
		int a;
		cin >> a;
		sVec[i] = { a,i };
	}
	sort(sVec.begin(), sVec.end());
	segment_tree st = segment_tree(inp);
	int ans = 0;
	for (size_t i = 0; i < n; i++)
	{
		st.update(sVec[i].second, 0);
		int beg = max(0ll, sVec[i].second - sVec[i].first);
		int end = min(n - 1, sVec[i].second + sVec[i].first);
		ans += st.sumRange(beg, end);
	}
	cout << ans << "\n";
	return 0;
}