#include "bits/stdc++.h"
#include <unordered_set>
#define rep(i,n) for(int i = 0; i < n; i++)
typedef long long ll;
typedef unsigned long long ull;
using namespace std;
#define vll vector<vector<long long>>
#define vl vector<long long>
#define vi vector<int>
#define vii vector<vector<int>>
#define pb push_back
#define pf push_front
#define ld long double
#define Sort(a) sort(a.begin(),a.end())
#define cSort(a,cmp) sort(a.begin(),a.end(),cmp)
#define reSort(a) sort(a.rbegin(), a.rend())
static const ll llMAX = numeric_limits<long long>::max();
static const int intMAX = numeric_limits<int>::max();
static const ll  llMIN = numeric_limits<long long>::min();
static const int intMIN = numeric_limits<int>::min();
static const ll d_5 = 100000;
static const ll d9_7 = 1000000007;
static const ll d_9 = 1000000000;
static const double PI=3.14159265358979323846;
template<class T>
T gcd(T a,T b){
  if(a==0){
    return b;

  }else if(b==0){
    return a;
  }
  while(1) {
    if(a < b) swap(a, b);
    if(!b) break;
    a %= b;
  }
  return a;
}
ll digitpower(ll a,ll b){//aのb乗を計算
  if(b==1){
    return a;
  }else if(b==0){
    return 1;
  }
  int mode=1;
  if(mode==0){
    if(b%2==1){
      ll tmp = digitpower(a,(b-1)/2);
      tmp%=d9_7;
      tmp*=tmp;
      tmp%=d9_7;
      tmp*=a;
      return (tmp)%d9_7;
    }else{
      ll tmp = digitpower(a,(b)/2);
      tmp%=d9_7;
      tmp*=tmp;
      tmp%=d9_7;
      return (tmp)%d9_7;
    }
  }else{
    if(b%2==1){
      ll tmp = digitpower(a,(b-1)/2);
      tmp*=tmp;
      tmp*=a;
      return (tmp);
    }else{
      ll tmp = digitpower(a,(b)/2);
      tmp*=tmp;
      return (tmp);
    }
  }
  return 0;
  
}
template<class T>
void Printvector(std::vector<T> &a){
  int size = a.size();
  rep(i,size){
    cout<<a[i]<<" ";
  }
  cout<<endl;
}
template<class T>
void Printvector(std::vector<std::vector<T> > &a){
  int size = a.size();
  rep(i,size){
    int size2=a[i].size();
    rep(j,size2){
      cout<<a[i][j]<<" ";
    }
    cout<<endl;
  }
  cout<<endl;
}
unordered_set<ll> primes(ll n){
  unordered_set<ll> ret;
  for(ll i=2;i*i<=n;i++){
    if(n%i==0){
      ret.insert(i);
      ret.insert(n/i);
    }
  }
  return ret;
}
long long modinv(long long a, long long m) {//modの逆元
    long long b = m, u = 1, v = 0;
    while (b) {
        long long t = a / b;
        a -= t * b; swap(a, b);
        u -= t * v; swap(u, v);
    }
    u %= m; 
    if (u < 0) u += m;
    return u;
}
struct c{
  ll a;
  ll b;
};
template<class T>
vector<T> getaccum(vector<T> a){
  int size=a.size();
  vector<T> ans(size);
  ans[0]=a[0];
  for(int i=0;i<size-1;i++){
    ans[i+1]=ans[i]+a[i+1];
    //ans[i+1]%=d9_7;
  }
  return ans;
}
// 比較関数を定義
bool cmp( const c& left, const c& right ) {
    return left.b>right.b;
}

int main(void){
  ll n;
  cin>>n;
  vl a(n);
  rep(i,n){
    cin>>a[i];
  }
  unordered_map<ll,ll> past;
  past.insert({-a[0],1});
  ll ans=0;
  for(ll now=1;now<n;now++){
    if(past.find(a[now]-now)!=past.end()){
      ans+=past[a[now]-now];
    }
    if(past.find(-(a[now]+now))!=past.end()){
      past[-(a[now]+now)]++;
    }else{
      past.insert({-(a[now]+now),1});
    }
  }
  cout<<ans<<endl;


  return 0;
}

//clang++ -g -fsanitize=address -fno-omit-frame-pointer 1.cpp -o test.exe

//<<std::setprecision(30)

//重複削除
 /* std::sort(vec.begin(), vec.end());
 
  vec.erase(std::unique(vec.begin(), vec.end()), vec.end());*/

  //ペアの全探索
  /*do{
		int s=0;
		for(int i=0;i<n/2;i++)s+=v[i]^v[i+n/2];
		r=max(s,r);
		reverse(v.begin()+n/2,v.end());
	}while(next_permutation(v.begin(),v.end()));*/


Measured memory_bytes: 12840