#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp>  
#include <functional>
#include <iostream> 
#include<bits/stdc++.h>
using namespace __gnu_pbds;  
using namespace std;  
// Declaring ordered_set 
typedef tree<long long int, null_type, less<long long int>, rb_tree_tag,  
            tree_order_statistics_node_update>  
    ordered_set;  
// https://www.geeksforgeeks.org/order_of_key-in-c/
 
typedef pair<long long int,long long int> pa;
 
#define resize(A) A.resize(unique(A.begin(), A.end()) - A.begin())
#define pb push_back
#define  MAX 200005

#define endl "\n"                   //Take care in case of interactive problems


long long int mod=998244353;
long long int MOD=1e9+7;
long long int INF=1e17;
 
 
 
 
////////////////////////////////////////////// sieve  //////////////////////////////////////////////////////////////////
/*
 bool C[MAX];
 
 void sieve()
 {
    long long int i,j;
    for(i=2;i<MAX;i++)
    {
      if(!C[i])
     { 
         
       
        for(j=2*i;j<MAX;j=j+i)
        {
            C[j]=true;
         
        }
      }
        
    }
     
 }*/
 
 
 
 
//////////////////////////////////////////////////////////D-S-U/////////////////////////////////////////////////////////
/*long long int parent[MAX];
long long int size[MAX];
void initialise()
{ //cout<<MAX<<" ";
    for(long  long int i=0;i<MAX;i++)
    {
        parent[i]=i;
        //cout<<i<<" ";
        size[i]=1;
    }
}
long long int  root(long long int x)
{
    while(x!=parent[x])
    {
        parent[x]=parent[parent[x]];
        x=parent[x];
    }
    return x;
}
 
    
void union1(long long int x,long long int y)
{
    if(size[x]>=size[y])
    {
        size[x]+=size[y];
        parent[y]=x;
        
    }
    else
    {
        size[y]+=size[x];
        parent[x]=y;
    }
}*/
//////////////////////////////////////////////////////MODPOWER//////////////////////////////////////////////////////////
 
 
/*long long int modulo(long long int a,long long  int b, long long int n){
    long long int  x=1, y=a%n; 
    while (b > 0) {
        if (b%2 == 1) {
            x = (x*y) % n; // multiplying with base
        }
        y = (y*y) % n; // squaring the base
        b /= 2;
    }
    return x % n;
}*/
 
 
////////////////////////////////////////////////BIT/////////////////////////////////////////////////////////////////////
 /*long long int bit[MAX+1]={0};
long long int query(long long int in,long long int n)
{
    //in=in+1;
    long long int sum=0;
    if(in==0)
        return 0;
    while(in>0)
    {
      sum=sum+bit[in];
        
        in=in-(in&(-in));
    }
    return sum;
}
void update(long long int in,long long int n,long long int val)
{
    // in=in+1;
    
    while(in<=n)
    {
       // cout<<in<<" ";
        bit[in]+=val;
        in=in+ (in &(-in));
    }
}
 
 
 
*/
 
//////////////////////////////////////////////// DFS //////////////////////////////////////////////////////////////////
/*
  bool visited[MAX];
 long long int parent[MAX];
 long long int dist[MAX];


void dfs(vector<long long int>adj[],long long int i,long long int k)
{
    visited[i]=true;
    for(auto j:adj[i])
    {
        if(!visited[j])
        {
            dist[j]=dist[i]+1;
            parent[j]=i;
            dfs(adj,j,k);
            
        }
        
    }
}
 */
 

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
 

 
 
//long long int R[4]={1,0,-1,0};
//long long int C[4]={0,1,0,-1};
 
 
//NEVER GONNA GIVE YOU UP, NEVER GONNA LET YOU DOWN! 


int main()
{
  ios_base::sync_with_stdio(false); 
  cin.tie(NULL);
  cout.tie(NULL);
long long int  sum,d,i,w,j,k=0,l=0,r=0,m,n,c,p=0,cnt=0,cnt1=0,x,y=0,z=0,a,b,low,mid,high,ans=0,ans1=0,maxi=0,mini=INF,t1,x1,y1,t,o,i1;
string s,s1,s2,s3;    long long qt,xxx;        char kk,aa,bb,jj;

   cin>>n;
    long long int A[n];
    for(i=0;i<n;i++)
    {
        cin>>A[i];
    }
    map<long long int,long long int>ma;
    ans=0;
    for(i=0;i<n;i++)
    {
        ans+=ma[i-A[i]];
        ma[i+A[i]]++;
    }
    cout<<ans<<endl;
 
}


Measured memory_bytes: 27808