#line 2 "header.hpp"

//%snippet.set('header')%
//%snippet.fold()%
#ifndef HEADER_H
#define HEADER_H

// template version 2.0
using namespace std;
#include <bits/stdc++.h>

// varibable settings
#define int long long
const int INF = 1e18;

#define _overload3(_1, _2, _3, name, ...) name
#define _rep(i, n) repi(i, 0, n)
#define repi(i, a, b) for (int i = (int)(a); i < (int)(b); ++i)
#define rep(...) _overload3(__VA_ARGS__, repi, _rep, )(__VA_ARGS__)
#define _rrep(i, n) rrepi(i, 0, n)
#define rrepi(i, a, b) for (int i = (int)((b)-1); i >= (int)(a); --i)
#define r_rep(...) _overload3(__VA_ARGS__, rrepi, _rrep, )(__VA_ARGS__)
#define each(i, a) for (auto &&i : a)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((int)(x).size())
#define pb(a) push_back(a)
#define mp(a, b) make_pair(a, b)
#define mt(...) make_tuple(__VA_ARGS__)
#define ub upper_bound
#define lb lower_bound
#define lpos(A, x) (lower_bound(all(A), x) - A.begin())
#define upos(A, x) (upper_bound(all(A), x) - A.begin())
template <class T>
inline void chmax(T &a, const T &b) {
    if ((a) < (b)) (a) = (b);
}
template <class T>
inline void chmin(T &a, const T &b) {
    if ((a) > (b)) (a) = (b);
}

#define divceil(a, b) ((a) + (b)-1) / (b)
#define is_in(x, a, b) ((a) <= (x) && (x) < (b))
#define uni(x)    \
    sort(all(x)); \
    x.erase(unique(all(x)), x.end())
#define slice(l, r) substr(l, r - l)

typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef long double ld;
typedef pair<int, int> pii;
typedef tuple<int, int, int> iii;

template <typename T>
using PQ = priority_queue<T, vector<T>, greater<T>>;
struct Fast {
    Fast() {
        std::cin.tie(0);
        ios::sync_with_stdio(false);
    }
} fast;
void check_input() {
    assert(cin.eof() == 0);
    int tmp;
    cin >> tmp;
    assert(cin.eof() == 1);
}

#if defined(PCM) || defined(LOCAL)
#else
#define dump(...) ;
#define dump_1d(...) ;
#define dump_2d(...) ;
#define cerrendl ;
#endif

#endif /* HEADER_H */
//%snippet.end()%
#line 2 "solve.cpp"

template <typename T = int>
struct bit {  //{{{
    int n;
    vector<T> dat;
    vector<T> raw;

    bit(int _n = 0) {  //{{{
        n = _n;
        dat = vector<T>(n);
        raw = vector<T>(n);
    }  //}}}

    bit(vector<T> a) {  // {{{
        n = (int)a.size();
        dat = vector<T>(n);
        raw = vector<T>(n);
        for (int i = 0; i < n; i++) {
            add(i, a[i]);
            raw[i] = a[i];
        }
    }  //}}}

    T _rsum(int i) {  //{{{ [0, i]
        T s = 0;
        while (i >= 0) {
            s += dat[i];
            i = (i & (i + 1)) - 1;
        }
        return s;
    }  //}}}

    T sum(int l, int r) {  //{{{  [l, r)
        if (l > r - 1) return 0;
        return _rsum(r - 1) - _rsum(l - 1);
    }  //}}}

    void add(int i, T x) {  //{{{
        raw[i] += x;
        while (i < n) {
            dat[i] += x;
            i |= i + 1;
        }
    }  //}}}

    int lower_bound(T x) {  // a[0]+...+a[ret] >= x{{{
        int ret = -1;
        int k = 1;
        while (2 * k <= n) k <<= 1;
        for (; k > 0; k >>= 1) {
            if (ret + k < n && dat[ret + k] < x) {
                x -= dat[ret + k];
                ret += k;
            }
        }
        return ret + 1;
    }  //}}}

    friend ostream& operator<<(ostream& os, bit<T>& b) {  //{{{
        os << endl << "  raw:" << b.raw << endl;
        vector<T> acum;
        rep(i, b.n) { acum.pb(b.sum(i)); }
        os << "  acm:" << acum << endl;
        return os;
    }  //}}}
};     //}}}

signed main() {
    //input
    int n;cin>>n;
    vector<int> a(n);
    rep(i, n) {
        cin>>a[i];
    }
    dump(a);
    check_input();

    //solve

    map<int, int> d;
    int ans = 0;
    rep(i, n){
        ans += d[a[i]-i];
        d[-a[i]-i]++;
    }
    cout << ans << endl;

    return 0; 
}


Measured memory_bytes: 27488