#include <iostream>
#include <string>
#include <vector>
#include <set>
#include <stack>
#include <queue>
#include <map>
#include <algorithm>
#include <iomanip>
#include <math.h>
#include <string.h>
#include <cstdio>
#include <tuple>
using namespace std; using ll = long long; using ld = long double;  using pll = pair<ll, ll>;
using vl = vector<ll>; using vll = vector<vl>; using vpll = vector<pll>;
const ld PI = 3.1415926535897932;
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#define rep1(n) for(ll i=0;i<n;++i)
#define rep2(i,n) for(ll i=0;i<n;++i)
#define rep3(i,a,b) for(ll i=a;i<b;++i)
#define rep4(i,a,b,c) for(ll i=a;i<b;i+=c)
#define rep(...) overload4(__VA_ARGS__,rep4,rep3,rep2,rep1)(__VA_ARGS__)
#define mp make_pair
#define mt make_tuple
#define INT(...) int __VA_ARGS__;in(__VA_ARGS__)
#define LL(...) ll __VA_ARGS__;in(__VA_ARGS__)
#define ULL(...) ull __VA_ARGS__;in(__VA_ARGS__)
#define STR(...) string __VA_ARGS__;in(__VA_ARGS__)
#define CHR(...) char __VA_ARGS__;in(__VA_ARGS__)
#define DBL(...) double __VA_ARGS__;in(__VA_ARGS__)
#define LD(...) ld __VA_ARGS__;in(__VA_ARGS__)
void in() {}
void debug_out() { cerr << endl; }
template<typename Head, typename... Tail>
void debug_out(Head h, Tail... t) {
	cerr << " " << h;
	if (sizeof...(t) > 0) cerr << " :";
	debug_out(t...);
}
template <typename T>
ostream& operator<<(ostream& os, vector<T> vec) {
	for (size_t i = 0; i < vec.size(); i++)os << vec[i] << (i + 1 == vec.size() ? "" : " ");
	return os;
}
ll ctoi(char c) {
	switch (c) {
	case '0': return 0; case '1': return 1; case '2': return 2;
	case '3': return 3; case '4': return 4; case '5': return 5;
	case '6': return 6; case '7': return 7; case '8': return 8;
	case '9': return 9;  default: return 0;
	}
}
void vout(vl V) {
	for (ll i = 0; i < (ll)V.size(); i++) cout << V[i] << " ";
	cout << endl;
}
bool pairCompare(const pll firstElof, pll secondElof)
{
	return firstElof.first > secondElof.first;
}
//**Snippetリスト**//
//rep, vin, all, iteLoop//
//bitSearch, bitList, nod, gcdlcm, isPrime, primeFactorize, Npow, divisor, modinv, dfs, bfs, eratos//
//dijkstra, WarshallFloyd, BellmanFord, UnionFind, COM, digitDP ,RMQ//
ll i, j, k, l; ll N, M, K, H, W, L, X, Y, Z, R, Q;
ll MOD = 1000000007; ll INF = 1LL << 62; ll ans = 0; ll z = 0, o = 1;
vl flag, color; vll path; vl A; vl dp; vl Ans;

int main() {
	cin >> N;
	vpll A(N);
	for (i = 0; i < N; i++) {
		ll a;
		cin >> a;
		a += N - i - 1;
		A[i] = mp(a, i+1);
	}
	sort(A.begin(), A.end());
	vl B(N);
	for (i = 0; i < N; i++) {
		B[i] = A[i].first;
	}
	for (i = 0; i < N; i++) {
		ll key = (N - A[i].second) * 2 - B[i];
		//debug(B, key,ans);
		if (B[N - 1] >= key) {
			ll left = -1; //「index = 0」が条件を満たすこともあるので、初期値は -1
			ll right = B.size(); // 「index = a.size()-1」が条件を満たさないこともあるので、初期値は V.size()
			while (right - left > 1) {
				ll mid = left + (right - left) / 2;
				if (B[mid] >= key) right = mid;
				else left = mid;
			}
			//debug(right);
			if (B[right] == key) {
				/* left は条件を満たさない最大の値、right は条件を満たす最小の値になっている */
				ll pre1 = right;
				if (B[N - 1] < key + 1) {
					ans += N - pre1;
				}
				else {
					ll left = -1; //「index = 0」が条件を満たすこともあるので、初期値は -1
					ll right = B.size(); // 「index = a.size()-1」が条件を満たさないこともあるので、初期値は V.size()
					while (right - left > 1) {
						ll mid = left + (right - left) / 2;
						if (B[mid] >= key + 1) right = mid;
						else left = mid;
					}
					//debug(right);
					ans += right - pre1;
				}
			}
		}
	}
	cout << ans;
}

Measured memory_bytes: 8012