#include <bits/stdc++.h>
using namespace std;

#define pii  pair<int, int>
#define vi vector<int >
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define x first
#define y second
#define sz(x) (x).size()
#define all(x) (x).begin(), (x).end() //Forward traversal
#define rall(x) (x).rbegin, (x).rend() //reverse traversal

#define ll long long
#define ld long double

const ld EPS = 10e-9;
const int INF = 1e9 + 5;
const long long INFLL = (ll)INF * (ll)INF;
const ll MOD = 1e9 + 7;
const ld PI = 3.14159265358979323846;

template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v)
{
    os << "[";
    for (int i = 0; i < (int)v.size(); ++i) {
        os << v[i];
        if (i != (int)v.size() - 1)
            os << ", ";
    }
    os << "]\n";
    return os;
}

template <typename T>
ostream& operator<<(ostream& os, const set<T>& v)
{
    os << "{";
    for (const auto& it : v) {
        os << it;
        if (it != *v.rbegin())
            os << ", ";
    }
    os << "}\n";
    return os;
}

template <typename T>
ostream& operator<<(ostream& os, const unordered_set<T>& v)
{
    os << "{";
    for (const auto& it : v) {
        os << it;
        if (it != *v.rbegin())
            os << ", ";
    }
    os << "}\n";
    return os;
}

template <typename T, typename S>
ostream& operator<<(ostream& os, const map<T, S>& v)
{
    os << "{";
    for (const auto& it : v) {
        os << it.first << " : "
           << it.second;

        if( it != *v.rbegin())
            os  << ", ";
    }

    os << "}\n";
    return os;
}

template <typename T, typename S>
ostream& operator<<(ostream& os, const unordered_map<T, S>& v)
{
    os << "{";
    for (const auto& it : v) {
        os << it.first << " : "
           << it.second;

        if( it != *v.rbegin())
            os  << ", ";
    }

    os << "}\n";
    return os;
}

template <typename T, typename S>
ostream& operator<<(ostream& os, const pair<T, S>& v)
{
    os << "(";
    os << v.first << ", "
       << v.second << ")";
    os << "\n";
    return os;
}

#define DEBUG

#ifdef DEBUG
#define debug(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1>
void __f(const char* name, Arg1&& arg1){
    cerr << name << " : " << arg1;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args){
    const char* comma = strchr(names + 1, ',');cerr.write(names, comma - names) << " : " << arg1<<" | ";__f(comma+1, args...);
}
#else
#define debug(...)
#endif


// __gcd(x, y)

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);

    int N;

    cin >> N;

    vector<ll> a(N+1);
    for(int i = 1; i <= N;i++)
        cin >> a[i];

    unordered_map<ll, ll> s1, s2;

    ll ma = 0;
    for(int i = 1; i <= N; i++)
    {
        ma = max(ma, i+a[i]);
        s1[i + a[i]]++;
        s2[i - a[i]]++;
    }

    ll num = 0;
    for(int i = 1; i <= N; i++)
    {
        if(s1.count(i) && s2.count(i))
            num += s1[i] * s2[i];
    }

    cout << num << endl;


//    vector<int> indices;
//    for(int i = 0; i <= N; i++)indices.pb(i);
//    sort(all(indices), [&](const auto& l, const auto& r) {return a[l] < a[r];});
//    sort(all(a));

//    ll num = 0;
//    for(int i = 1; i <= N; i++)
//    {
//        int b = 1, e = N;
//        while(b <= e)
//        {
//            int mid = (b + e) / 2;

//            if(a[i] + a[mid] == abs(indices[i]-indices[mid]))
//            {
//                num++;
//                break;
//            }
//            else if(a[i] + a[mid] > abs(indices[i]-indices[mid]))
//            {
//                e = mid - 1;
//            }
//            else
//            {
//                b = mid + 1;
//            }
//        }
//    }

//    cout << num << endl;

    return 0;
}




Measured memory_bytes: 20080