#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;

template<typename T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

int main() {
	int N;
	cin >> N;
	vector<int> v(N + 1);
	vector<vector<int>> u(N + 1);
	vector<vector<int>> w(N + 1);
	for (int i = 1; i <= N; i++) {
		cin >> v[i];
		u[max(1, i - v[i])].push_back(i);
		w[min(N, i)].push_back(i);
	}
	ordered_set<int> s;
	long long x = 0;
	for (int i = 1; i <= N; i++) {
		for (int j : u[i]) {
			s.insert(j);
		}
		x += s.order_of_key(min(N + 1, i + v[i] + 1)) - s.order_of_key(max(1, i - v[i])) - 1;
		for (int j : w[i]) {
			s.erase(j);
		}
	}
	cout << x << endl;
	return 0;
}