#include <bits/stdc++.h>
using namespace std;
using llong = long long int;

#define rep(i, n) for (int i = 0; i < ((int)n); ++i)
#define stl_rep(itr, x) for (auto itr = x.begin(); itr != x.end(); ++itr)
#define all(x) x.begin(), x.end()
#define allr(x) x.rbegin(), x.rend()

const static int MOD = 1000000007;
const static int INF = 1000000000;
const static int dx[4] = {1, 0, -1, 0};
const static int dy[4] = {0, 1, 0, -1};

template <class Abel>
struct BIT
{
    const Abel UNITY_SUM = 0; // to be set
    vector<Abel> dat;

    /* [1, n] */
    BIT(int n) : dat(n + 1, UNITY_SUM) {}
    void init(int n) { dat.assign(n + 1, UNITY_SUM); }

    /* a is 1-indexed */
    inline void add(int a, Abel x)
    {
        for (int i = a; i < (int)dat.size(); i += i & -i)
            dat[i] = dat[i] + x;
    }

    /* [1, a], a is 1-indexed */
    inline Abel sum(int a)
    {
        Abel res = UNITY_SUM;
        for (int i = a; i > 0; i -= i & -i)
            res = res + dat[i];
        return res;
    }

    /* [a, b), a and b are 1-indexed */
    inline Abel sum(int a, int b)
    {
        return sum(b - 1) - sum(a - 1);
    }

    /* debug */
    void print()
    {
        for (int i = 1; i < (int)dat.size(); ++i)
            cout << sum(i, i + 1) << ",";
        cout << endl;
    }
};

template<typename T>
struct UnionFind {
    vector<int> par;
    vector<T> sz;

    UnionFind(int n) {
        par.resize(n);
        sz.resize(n);
        for (int i = 0; i < n; ++i) {
            par[i] = i;
            sz[i] = 1;
        }
    }

    int find_root(int x) {
        return par[x] == x ? x : par[x] = find_root(par[x]);
    }

    bool same(int x, int y) {
        return find_root(x) == find_root(y);
    }

    void unite(int x, int y) {
        x = find_root(x);
        y = find_root(y);
        if (x == y) return;
        
        if (sz[x] < sz[y]) swap(x, y);
        sz[x] += sz[y];
        par[y] = x;
    }

    T size_of_tree(int x) {
        return sz[find_root(x)];
    }
};
/* --------------- template end --------------- */

/* --------------- function start --------------- */
llong my_gcd(llong a, llong b) {
    return (a ? my_gcd(b % a, a) : b);
}
llong my_lcm(llong a, llong b) {
    return a * b / my_gcd(a, b);    
}

llong facmod(llong n) {
    if (n == 0) return 1;

    llong res = n;
    for (int i = n - 1; i > 0; --i) {
        res = (res * i) % MOD;
    }

    return res;
}
llong powmod(llong a, llong n) {
    llong res = 1;
    while (n > 0) {
        if (n & 1) res = (res * a) % MOD;
        a = (a * a) % MOD;
        n >>= 1;
    }

    return res;
}
llong modinv(llong n) {
    return powmod(n, MOD - 2);
}
llong combmod(llong n, llong k) {
    llong res = 1;
    for (int i = 0; i < k; ++i) {
        res = (res * (n - i)) % MOD;
    }
    res = (res * modinv(facmod(k))) % MOD;

    return res;
}

vector<llong> divisors(llong n) {
    vector<llong> ret;
    for(llong i = 1; i * i <= n; i++) {
        if(n % i == 0) {
            ret.push_back(i);
            if((i * i) != n) ret.push_back(n / i);
        }
    }
    sort(begin(ret), end(ret));

    return ret;
}
/*
bool primeNumber(llong n){
    if(n < 2LL) return false;
    else{
        for(llong i = 2LL; i * i <= n; i++){
            if(n % i == 0LL) return false;
        }
        return true;
    }
} */
/* --------------- function end --------------- */

//////////////////////////////////////////////////////////////////////////////////////
int main (int argc, char *argv[]) {
    cin.tie(0);
    ios::sync_with_stdio(false);

    int n;
    cin >> n;
    vector<llong> A(n);
    rep(i, n) cin >> A[i];

    vector<llong> Ajmj(n);
    rep(i, n) Ajmj[i] = A[i] - (i + 1);
    vector<llong> mAipi(n);
    rep(i, n) mAipi[i] = -(A[i] + i + 1);
    sort(all(Ajmj));
    sort(all(mAipi));

    llong ans = 0;
    rep(i, n) {
        ans += upper_bound(all(mAipi), Ajmj[i]) - lower_bound(all(mAipi), Ajmj[i]);
    }

    cout << ans << endl;
    
    return 0;
}
//////////////////////////////////////////////////////////////////////////////////////


Measured memory_bytes: 7960