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

template <class T> ostream& operator << (ostream &os, const vector<T> &v) { for (T i : v) os << i << ' '; return os; }
template <class T> ostream& operator << (ostream &os, const set<T> &v) { for (T i : v) os << i << ' '; return os; }
template <class T, class S> ostream& operator << (ostream &os, const pair<T, S> &v) { os << v.first << ' ' << v.second; return os; }
template <class T, class S> ostream& operator << (ostream &os, const map<T, S> &v) { for (auto i : v) os << '(' << i.first << "=>" << i.second << ')' << ' '; return os; }
template <class T, class S> ostream& operator << (ostream &os, const unordered_map<T, S> &v) { for (auto i : v) os << '(' << i.first << "=>" << i.second << ')' << ' '; return os; }
 
#ifndef ONLINE_JUDGE
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
    template <class Arg1> void __f(const char* name, Arg1&& arg1) { cerr << name << " : " << arg1 << endl; }
    template <class Arg1, class... Args>
    void __f(const char* names, Arg1&& arg1, Args&&... args) {
        const char* sep = strchr(names + 1, ',');
        cerr.write(names, sep - names) << " : " << arg1 << "  ";
        __f(sep + 1, args...);
    }
#else
#define trace(...) 0
#pragma GCC optimize ("O3")
#pragma GCC optimize ("unroll-loops")
#pragma GCC target("avx2,sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#define _CRT_SECURE_NO_WARNINGS
#endif

#define fast_cin() ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define MAX 1000000
#define ll long long
#define db double
#define str string
#define pb push_back
#define For(i,s,e) for (ll i=(s); i<(e); i++)
#define Forrev(i,s,e) for (ll i=(s); i>=(e); i--)
#define all(v) v.begin(),v.end()

#define vll vector<ll>
#define vs vector<string>
#define mapll map<ll,ll>
#define pll pair<ll,ll>
#define initialise(a, x) memset(a, x, sizeof(a))
#define maxheap priority_queue<ll>
#define minheap priority_queue<ll,vector<ll> ,greater<ll>>

#define ff first
#define ss second
#define endl "\n"
#define mp make_pair
const ll mod=1e9 + 7;

ll takemod(ll a) {
    return ((a%mod)+mod)%mod;
}

ll gcd(ll a, ll b) { 
    if (b == 0) 
        return a; 
    return gcd(b, a % b);        
}

ll fast_exp(ll base, ll expo) {
    ll res=1;
    while(expo>0) {
        if(expo&1) res=(res*base)%mod;
        base=(base*base)%mod;
        expo>>=1;}
    return res;
}

ll modinv(ll a) {
    return takemod(fast_exp(takemod(a), mod-2));
}

void solve()
{
    ll n;
    cin>>n;

    ll arr[n];
    For(i,0,n)
        cin>>arr[i];

    map<ll,ll> mp1;
    map<ll,ll> mp2;
    For(i,0,n)
    {
        mp1[i+1-arr[i]]++;
        mp2[arr[i]+i+1]++;
    }

    ll ans=0;
    For(i,0,n)
    {
        ll val1=i+1-arr[i];
        if(mp2.find(val1)!=mp2.end())
            ans+=(mp2[val1]);

        val1=arr[i]+i+1;
        if(mp1.find(val1)!=mp1.end())
            ans+=mp1[val1];
    }

    cout<<ans/2<<endl;

}

signed main() 
{
    fast_cin();
    // #ifndef ONLINE_JUDGE
    // freopen("C:/IO/in.txt", "r", stdin);
    // freopen("C:/IO/out.txt", "w", stdout);
    // #endif
    
    int t=1;
    //cin>>t;
    while(t--)
    {
        solve();
    }
    
    
    return 0;    
}

Measured memory_bytes: 28272