#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
typedef long long ll;
typedef long double lld;
typedef long long int lli;
using namespace std;
const int N = 1000001;
const int MOD = 1e9+7;
const bool DEBUG = 1;
#define sd(x) scanf("%d ", &x)
#define sd2(x, y) scanf("%d%d", &x, &y)
#define sd3(x, y, z) scanf("%d%d%d", &x, &y, &z)
#define endl "\n"
#define fi first
#define se second
#define eb emplace_back
#define fbo find_by_order
#define ook order_of_key
#define pb(x) push_back(x)
#define mp(x, y) make_pair(x, y)
#define LET(x, a) __typeof(a) x(a)
#define foreach(it, v) for (LET(it, v.begin()); it != v.end(); it++)
#define MEMS(a, b) memset(a, b, sizeof(a))
#define _			\
    ios_base::sync_with_stdio(false);			\
    cin.tie(NULL);			\
    cout.tie(NULL);
#define __			\
    freopen("input.txt", "r", stdin);			\
    freopen("output.txt", "w", stdout);
#define all(c) c.begin(), c.end()
#define inf 1000000000000000001
#define epsilon 1e-6
#define int ll
#define RUN_T			\
    int _t;			\
    cin >> _t;			\
    while (_t--)
#define tr(...)			\
if (DEBUG) {			\
    cout << __FUNCTION__ << ' ' << __LINE__ << " = ";			\
    trace(#__VA_ARGS__, __VA_ARGS__);			\
}
template <typename S, typename T>
ostream &operator<<(ostream &out, pair<S, T> const &p) {
    out << '(' << p.fi << ", " << p.se << ')';
    return out;
}
template <typename T>
ostream &operator<<(ostream &out, set<T> const &v) {
    for (auto i = v.begin(); i != v.end(); i++)
        out << (*i) << ' ';
    return out;
}
template <typename T, typename V>
ostream &operator<<(ostream &out, map<T, V> const &v) {
    for (auto i = v.begin(); i != v.end(); i++)
        out << "\n" << (i->first) <<  ":"  << (i->second);
    return out;
}
template <typename T, typename V>
ostream &operator<<(ostream &out, unordered_map<T, V> const &v) {
    for (auto i = v.begin(); i != v.end(); i++)
        out << "\n" << (i->first) <<  ":"  << (i->second);
    return out;
}
template <typename T>
ostream &operator<<(ostream &out, multiset<T> const &v) {
    for (auto i = v.begin(); i != v.end(); i++)
        out << (*i) << ' ';
    return out;
}
template <typename T>
ostream &operator<<(ostream &out, unordered_set<T> const &v) {
    for (auto i = v.begin(); i != v.end(); i++)
        out << (*i) << ' ';
    return out;
}
template <typename T>
ostream &operator<<(ostream &out, unordered_multiset<T> const &v) {
    for (auto i = v.begin(); i != v.end(); i++)
        out << (*i) << ' ';
    return out;
}
template <typename T> ostream &operator<<(ostream &out, vector<T> const &v) {
    ll l = v.size();
    for (ll i = 0; i < l - 1; i++)
        out << v[i] << ' ';
    if (l > 0)
        out << v[l - 1];
    return out;
}
template <typename T> void trace(const char *name, T &&arg1) {
    cout << name <<  ":"  << arg1 << endl;
}
template <typename T, typename... Args>
void trace(const char *names, T &&arg1, Args &&... args) {
    const char *comma = strchr(names + 1, ',');
    cout.write(names, comma - names) <<  ":"  << arg1 <<  "|" ;
    trace(comma + 1, args...);
}
template <typename T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
int gcd(int a, int b) {
     if (b == 0)
           return a;
     return gcd(b, a % b);
}
int32_t main(){
    int n;
    cin >> n;
    vector<int>v(n + 1, 0),a1(n+1, 0), a2(n+1, 0);
    for(int i = 1 ; i <= n; i++){
        cin >> v[i];
    }
    for(int i = 1; i <= n; i++){
        a1[i] = v[i] + i;
        a2[i] = i - v[i] ;
    }
    int ans = 0;
    map<int,int>mt;
    for(int j = n ; j >= 1; j--){
        ans += mt[a1[j]];
        mt[a2[j]]+=1;
    }
    // tr(mt);
    cout << ans << endl;
}

Measured memory_bytes: 30628