/**
 *    author:	raghuvanshraj
 *    created:	03.05.2020 06:00:28 PM
**/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>

#define MOD 1000000007
#define MAX 1000000000
#define MIN -1000000000

#define SET_ARR(arr,n,val) for (int i = 0; i < n; ++i) arr[i] = val
#define SET_ARR2D(arr,n,m,val) for (int i = 0; i < n; ++i) for (int j = 0; j < m; ++j) arr[i][j] = val
#define INPUT_ARR(arr,n) for (int i = 0; i < n; ++i) cin >> arr[i];
#define INPUT_ARR2D(arr,n,m) for (int i = 0; i < n; ++i) for (int j = 0; j < m; ++j) cin >> arr[i][j];
#define PRINT_ARR(arr) for (auto x : arr) cout << x << ' '; cout << endl
#define PRINT_ARR2D(arr) for (auto x : arr) { for (auto y : x) cout << y << ' '; cout << endl; }

#define ALL(v) v.begin(), v.end()
#define RALL(v) v.rbegin(), v.rend()

using namespace std;
using namespace __gnu_pbds;

typedef long long LL;
typedef unsigned long long ULL;
typedef long double LD;

template <typename X, typename T>
auto vectors(X x, T a) {
	return vector<T>(x, a);
}

template <typename X, typename Y, typename Z, typename... Zs>
auto vectors(X x, Y y, Z z, Zs... zs) {
	auto cont = vectors(y, z, zs...);
	return vector<decltype(cont)>(x, cont);
}

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

template <typename T>
using max_heap = priority_queue<T>;

template <typename T>
using min_heap = priority_queue<T, vector<T>, greater<T>>;

template <
	typename T,
	typename = typename enable_if<is_arithmetic<T>::value, T>::type,
	typename U,
	typename = typename enable_if<is_arithmetic<U>::value, U>::type
>
pair<T,U> operator+(const pair<T,U> &a, const pair<T,U> &b) {
	return {a.first + b.first, a.second + b.second};
}

template <
	typename T,
	typename = typename enable_if<is_arithmetic<T>::value, T>::type,
	typename U,
	typename = typename enable_if<is_arithmetic<U>::value, U>::type
>
pair<T,U> operator-(const pair<T,U> &a, const pair<T,U> &b) {
	return {a.first - b.first, a.second - b.second};
}

template <
	typename T,
	typename = typename enable_if<is_arithmetic<T>::value, T>::type,
	typename U,
	typename = typename enable_if<is_arithmetic<U>::value, U>::type,
	typename V,
	typename = typename enable_if<is_arithmetic<V>::value, V>::type
>
pair<T,U> operator*(const V &a, const pair<T,U> &b) {
	return {a * b.first, a * b.second};
}

template <
	typename T,
	typename = typename enable_if<is_arithmetic<T>::value, T>::type,
	typename U,
	typename = typename enable_if<is_arithmetic<U>::value, U>::type,
	typename V,
	typename = typename enable_if<is_arithmetic<V>::value, V>::type
>
pair<T,U> operator*(const pair<T,U> &b, const V &a) {
	return {a * b.first, a * b.second};
}

template <
	typename T,
	typename = typename enable_if<is_arithmetic<T>::value, T>::type,
	typename U,
	typename = typename enable_if<is_arithmetic<U>::value, U>::type
>
istream& operator>>(istream &input, pair<T,U> &b) {
	input >> b.first >> b.second;
	return input;
}

template <
	typename T,
	typename = typename enable_if<is_arithmetic<T>::value, T>::type,
	typename U,
	typename = typename enable_if<is_arithmetic<U>::value, U>::type
>
ostream& operator<<(ostream &output, pair<T,U> &b) {
	output << b.first << ' ' << b.second;
	return output;
}

int main(int argc, char const *argv[]) {
	ios::sync_with_stdio(false);
	cin.tie(0);
	
	int n;
	cin >> n;
	vector<LL> arr(n);
	INPUT_ARR(arr,n);

	map<LL,LL> cnt;
	cnt[arr[0]]++;
	LL ans = 0LL;
	for (int i = 1; i <= n-1; ++i) {
		ans += cnt[i-arr[i]];
		cnt[arr[i]+i]++;
	}

	cout << ans;
	
	return 0;
}

Measured memory_bytes: 27424