#include <bits/stdc++.h>

// definitions
#define F first
#define S second
#define all(x) x.begin(), x.end()
#define clr(x) memset(x, 0, sizeof(x))
#define rep(i, l, r) for (long long i = (l); i < (long long)(r); i++)
#define repb(i, r, l) for (long long i = (r); i > (long long)(l); i--)
#define pb push_back
#define pf push_front
#define mod1 998244353
#define mod 1000000007
#define MAX_CHAR 256


using namespace std;
typedef vector<long long> vl;
typedef vector<char> vc;
typedef vector<string> vs;
typedef vector< pair<long long, long long> > vp;
typedef vector<tuple<long long , long long , long long> > vtup;
typedef deque <long long> dql;
typedef deque <char> dqc;
typedef long double ld;
typedef long long ll;
typedef unsigned long long ull;


// prevent collisions in unordered_map
struct custom_hash {
    static uint64_t splitmix64(uint64_t x) {
        x += 0x9e3779b97f4a7c15;
        x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
        x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
        return x ^ (x >> 31);
    }

    size_t operator()(uint64_t x) const {
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return splitmix64(x + FIXED_RANDOM);
    }
};

typedef unordered_map <long long, long long, custom_hash> umap;


// standard functions
ll gcd(ll x, ll y) {
    if (x < y) return gcd(y, x);
    if (y == 0) return x;
    return gcd(y, x % y);
} 

ll mystoi(string str)
{
    stringstream ss(str);
    ll ans=0;
    ss >> ans;

    return ans;
}


ll exp(ll x, ll ex) {
    ll ans = 1ll;
    while (ex > 0) {
        if (ex & 1ll) ans = (ans * x);
        ex >>= 1ll;
        x = (x * x);
    }
    return ans;
} 
 
ll nCrModp(ll n, ll r, ll p) 
{ 
    ll C[r+1]; 
    memset(C, 0, sizeof(C)); 
  
    C[0] = 1; 
    for (ll i = 1; i <= n; i++) 
    { 
        for (ll j = min(i, r); j > 0; j--) 
            C[j] = (C[j] + C[j-1])%p; 
    } 
    return C[r]; 
}
 
bool isPrime(ll n) 
{ 
    if (n <= 1)  return false; 
    if (n <= 3)  return true;  
    if (n%2 == 0 || n%3 == 0) return false; 
  
    for (ll i=5; i*i<=n; i=i+6) 
        if (n%i == 0 || n%(i+2) == 0) 
           return false; 
  
    return true; 
} 
 
vl countFreq(ll arr[], ll n) 
{ 
    unordered_map<ll, ll> mp; 
    vector <ll> v;
    
    for (ll i = 0; i < n; i++) 
        mp[arr[i]]++; 
   
    for (auto x : mp) 
        v.pb(x.second);
    
    return v;
}

ll binarySearch(ll arr[], ll l, ll r, ll x) 
{ 
    while (l <= r) 
    { 
        ll m = l + (r - l) / 2; 
        if (arr[m] == x) 
            return m; 
        if (arr[m] < x) 
            l = m + 1; 
        else
            r = m - 1; 
    }
    return -1; 
}

bool sort_cond(const pair<ll,ll> &a, const pair <ll,ll> &b)
{
    return a.second > b.second;
}

const ll MAXSIEVE=(1e3);
ll spf[MAXSIEVE];
void sieve() 
{ 
    spf[1] = 1; 
    for (ll i=2; i<MAXSIEVE; i++) 
        spf[i] = i; 

    for (ll i=4; i<MAXSIEVE; i+=2) 
        spf[i] = 2; 
  
    for (ll i=3; i*i<MAXSIEVE; i++) 
    { 
        if (spf[i] == i) 
        { 
            for (ll j=i*i; j<MAXSIEVE; j+=i) 
                if (spf[j]==j) 
                    spf[j] = i; 
        } 
    } 
}


// code beigns in main
int main()
{

    #ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    #endif
    
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    ll t=1;
    // cin >> t;
    
    while(t--)
    {
        ll n;
        cin >> n;

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

        ll imai[n];
        rep(i,0,n)
        imai[i]=i+1-arr[i];

        ll jpaj[n];
        rep(j,0,n)
        jpaj[j]=j+1+arr[j];

        map <ll,ll> m1;
        map <ll,ll> m2;

        rep(i,0,n)
        {
            m1[imai[i]]=0;
            m2[jpaj[i]]=0;
        }

        rep(i,0,n)
        {
            m1[imai[i]]++;
            m2[jpaj[i]]++;
        }

        ll tempi[n];
        ll tempj[n];
        rep(i,0,n)
        {
            tempi[i]=imai[i];
            tempj[i]=jpaj[i];
        }

        sort(tempi,tempi+n);
        sort(tempj,tempj+n);

        ll count1=0;
        rep(i,0,n)
        {
            if(binary_search(tempi,tempi+n,jpaj[i]))
                count1+=m1[jpaj[i]];
        }

        ll count2=0;
        rep(i,0,n)
        {
            if(binary_search(tempj,tempj+n,imai[i]))
                count2+=m2[imai[i]];
        }

        cout << max(count1,count2) << endl;
    }
}


Measured memory_bytes: 34616