Problem:
Score: 500 points

Problem Statement

You are the top spy of AtCoder Kingdom. To prevent the stolen secret from being handed to AlDebaran Kingdom, you have sneaked into the party where the transaction happens.

There are N attendees in the party, and they are given attendee numbers from 1 through N. The height of Attendee i is A_i.

According to an examination beforehand, you know that a pair of attendees satisfying the condition below will make the transaction.

The absolute difference of their attendee numbers is equal to the sum of their heights.

There are \frac{N(N-1)}{2} ways to choose two from the N attendees and make a pair. Among them, how many satisfy the condition above?

P.S.: We cannot let you know the secret.

Constraints

All values in input are integers.

2 \leq N \leq 2 \times 10^5

1 \leq A_i \leq 10^9\ (1 \leq i \leq N)

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 \dots A_N

Output

Print the number of pairs satisfying the condition.

Sample Input 1

6
2 3 3 1 3 1

Sample Output 1

3

A_1 + A_4 = 3, so the pair of Attendee 1 and 4 satisfy the condition.

A_2 + A_6 = 4, so the pair of Attendee 2 and 6 satisfy the condition.

A_4 + A_6 = 2, so the pair of Attendee 4 and 6 satisfy the condition.

No other pair satisfies the condition, so you should print 3.

Sample Input 2

6
5 2 4 2 8 8

Sample Output 2

0

No pair satisfies the condition, so you should print 0.

Sample Input 3

32
3 1 4 1 5 9 2 6 5 3 5 8 9 7 9 3 2 3 8 4 6 2 6 4 3 3 8 3 2 7 9 5

Sample Output 3

22

Solution in C++ (GCC 9.2.1) (judge verdict: Accepted)

/********************************
* AUTHOR:  ARNAB SEN             *
* NICK  :  arnab1729             *
* INSTITUTION: IIEST, SHIBPUR    *
********************************/

#include<bits/stdc++.h>

using namespace std;

using LL  = long long;
using LD  = long double;
using ULL = unsigned long long;
using VI   = vector<int>;
using VLL = vector<long long>;
using VULL= vector<unsigned long long>;
using VS  = vector<string>;
using VB  = vector<bool>;
using VC  = vector<char>;
using PII = pair<int, int>;
using PLL = pair<long long, long long>;
using VPII = vector<pair<int, int>>;
using VPLL = vector<pair<long long, long long>>;
using VPULL = vector<pair<unsigned long long, unsigned long long>>;
using VVI = vector<vector<int>>;
using VVLL = vector<vector<long long>>;
using MII = map<int, int>;
using MLL = map<long long, long long>;
using SLL = set<long long>;
using SI = set<int>;
using PQI = priority_queue<int>;

#define FF                             first
#define SS                             second
#define PB                             push_back
#define PF                             push_front
#define MP                             make_pair
#define all(a)                         (a).begin(), (a).end()
#define dec(n)                         cout<<fixed<<setprecision(n);
#define f(i, n)                        for(LL i=0; i<(LL)n; i++)
#define fr(i,n)                        for(LL i=(LL)(n-1); i>=0; i--)
#define fa(i, a, n)                     for(LL i=(LL)a; i<(LL)n; i++)
#define fra(i, a, n)                    for(LL i=(LL)(n-1); i>=a; i--)
#define fsort(a)                       sort(a.begin(), a.end())
#define rsort(a)                       sort(a.rbegin(), a.rend())
#define out(a)                         for(auto x: a) cerr<<x<<" "; debug();



#define fast ios_base::sync_with_stdio(false),cin.tie(nullptr),cout.tie(nullptr);srand(time(NULL))

template<class T>  T maxT(T a, T b){return (a>b?a:b);}
template<class T>  T minT(T a, T b){return (a<b?a:b);}
template<class T>  void setmax(T &a, T b){a=(a>b ?a:b);}
template<class T>  void setmin(T &a, T b){a=(a<b ?a:b);}

void fileio(){
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    freopen("debug.txt", "w", stderr);
}

void ofileio(){
  freopen("input.txt","r", stdin);
  freopen("output.txt", "w", stdout); 
}

void clock_out(){
  cerr<<"\nTime Elapsed : " << 1.0*clock() / CLOCKS_PER_SEC << " s\n";
}


void debug_out() { cerr << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << H;
  debug_out(T...);
}

template<typename... Args>
void readln(Args&... args)
{
    ((cin >> args), ...);
}

template<typename... Args>
void writeln(Args... args)
{
    ((cout << args << " "), ...);
}

 
#ifndef ONLINE_JUDGE
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#define fio() fileio()
#define ofio ofileio()
#define clock() clock_out()
#else
#define debug(...) 1
#define fio() 1
#define ofio() ofileio()
#define clock() 1
#endif

const LL MOD     = 1000000007;
const LL SIZE    = 100000;
const int INF    = 0x3f3f3f3f;
const LL LL_INF  = 0x3f3f3f3f3f3f3f3f;
const LD PI      = acos(-1);
const LL MAXN    = numeric_limits<LL>::max();

#define sz(x) (LL)(x).size()

void solve(){
   int n;
   cin>>n;
   VLL a(n);
   map<LL, LL> mp;
   f(i, n){
    cin>>a[i];
    mp[a[i]+i]++;
   }
   LL ans=0;
   f(i, n){
      LL val = i-a[i];
      ans+= mp[val];
   }

   cout<<ans;
}

int main(){
  fast;
 // fio();

  LL t=1;
  //cin>>t;
  while(t--){
      solve();
  }
  clock();
  return 0;
}