#ifdef LOCAL
#include <cstdio>
#include <vector>
#include <algorithm>
#include <iostream>
#include <cstring>
#include <math.h>
#include <map>
#else
#include <bits/stdc++.h>
#endif
using namespace std ;

typedef long long ll ;
typedef unsigned long long ull ;
#define rep(i , n ) for ( int i =0; i < n ; i++)
#define rrep(i , n) for ( int i = n-1; i >=0 ; i--)

#define _sort(arg) sort(begin(arg), end(arg))
#define MOD 1000000007
#define pb push_back
#define DEBUG(x) cout << #x << ": " << x << endl;

class UnionFind {
private:
	ull size_; 
	std::vector<ull> par, rank, count;

public:
	UnionFind() : size_(0), par(std::vector<ull>()), rank(std::vector<ull>()), count(std::vector<ull>()) {};
	UnionFind(ull size__) : size_(size__) {
		par.resize(size_); rank.resize(size_); count.resize(size__);
		for (ull i = 0; i < size_; i++) par[i] = i, rank[i] = 0, count[i] = 1;
	}
	ull size() { return size_; }
	// ull root(ull x) { return par[x] == x ? x : par[x] = root(par[x]); }
	
	ull root(ull x)
	{
		while(par[x] != x)
		{
			par[x] = par[par[x]];
			x = par[x];
		}
		return x;
	}
	
	bool same(ull x, ull y) { return root(x) == root(y); }

	void unite(ull x, ull y) {
		x = root(x), y = root(y);
		if (x == y) return;
		if (rank[x] < rank[y]) par[x] = y, count[y]+=count[x], count[x]=0;
		else if (rank[x] == rank[y]) par[y] = x, rank[x]++, count[x]+=count[y], count[y]=0;
		else par[y] = x, count[x] += count[y], count[y] = 0;
	}

	ull size__(ull x)
	{
		return count[root(x)];
	}

	bool operator==(const UnionFind &u) { return par == u.par; }
	bool operator!=(const UnionFind &u) { return par != u.par; }
};
/*
maybe sth wrong 
got wa in one problem
*/
ll modpow(ll a, ll b)
{
	if (b == 0)
	{
		return 1;
	}

	if (b % 2)
	{
		return a * modpow(a, b-1) % MOD;
	}
	else
	{
		ll halfB = b / 2;
		ll half = modpow(a, halfB);
		return half * half % MOD;
	}
}

ll calcComb(ll n, ll m)
{
	if (m > n-m)
	{
		return calcComb(n, n-m);
	}

	ll ansMul = 1;
	ll ansDiv = 1;

	rep(i, m)
	{
		ansMul *= (n-i);
		ansDiv *= (i+1);
		ansMul %= MOD;
		ansDiv %= MOD;
	}

	return ansMul * modpow(ansDiv, MOD - 2) % MOD;
}

__attribute__((constructor))
void initial()
{
	cin.tie(NULL);
	ios::sync_with_stdio(false);
}


int main()
{
	#if 0
	int N;
	cin >> N;
	ll A[N];
	ll L[N];
	ll R[N];
	
	map<ll, vector<ll> > mr;

	rep(i, N)
	{
		cin >> A[i];
		L[i] = i + A[i];
		R[i] = i - A[i];
		//mr[R[i]] = 
		if (mr.find(R[i]) == mr.end())
		{
			vector<ll> a;
			a.push_back(i);
			mr[R[i]] = a;
			cout << "pushed q=" << i << endl;
		}
		else
		{
			mr[R[i]].push_back(i);
			cout << "pushed =" << i << "; R[i]=" << R[i] << endl;
		}
	}

	int ret = 0;
	rep(i, N)
	{
		if (mr.find(L[i]) != mr.end())
		{
			//ret += mr[L[i]].size();
			auto tmp = mr[L[i]];
			sort(tmp.begin(), tmp.end());
			for (int j = 0; j < tmp.size(); j++)
			{
				cout << j << " " << tmp[j] << endl;
			}
			int v= (lower_bound(tmp.begin(), tmp.end(), i) - tmp.begin());
			ret += v;
			cout << "i=" << i << "; L[i]=" << L[i] << ", v=" << v << endl;
			
		}
	}
	cout << ret << endl;
	#endif
	ll ans = 0;
	ll n;cin>>n;
    vector<ll> a(n);
    map<ll,int> mp;
    rep(i,n) {
        cin>>a[i];
        ans+=mp[i-a[i]];
        mp[a[i]+i]++;
    }
    cout << ans << endl;

	return 0;
}


Measured memory_bytes: 27384