#include<bits/stdc++.h>
using namespace std;
#define	 fifo ios::sync_with_stdio(false); cin.tie(0);cout.tie(0)
#define  inf 2e18
#define  bg begin()
#define  en end()
#define  ll long long int
#define  pb push_back
#define  mp make_pair
#define  mod 1000000007
#define  vi vector<int>
#define  eb emplace_back
#define  lb lower_bound
#define  ub upper_bound
#define  bs binary_search
#define  vll vector<long long int>
#define  max3(a,b,c) max(a,max(b,c))
#define  min3(a,b,c) min(a,min(b,c))
#define  w(t) ll t; cin>>t; while(t--)
#define  forn(i,n) for(int i=0;i<n;i++)
#define  mem(a,val) memset(a,val,sizeof(a))
#define  Sort(vec) sort(vec.begin(),vec.end())
#define  RSort(vec) sort(vec.rbegin(),vec.rend())
#define  vin(v,n) for(int i=0;i<n;i++){cin>>x; v.pb(x); }

#define PI 3.141592653589793238462643383279502884L
ll Ceil(ll a,ll b){return a/b+(a%b!=0);}
ll fastModExp(ll a,ll b){ ll res=1; while(b>0) { if(b&1)res=(res*a)%mod; a=(a*a)%mod; b=b>>1; }return res;}

//: Binary Search,Greedy,dp,dfs and similar

/*
ll GC(ll a,ll b){ if(b==0)return a; else  return GC(b,a%b);}
ll inv_gc(ll a,ll b){return a/GC(a,b)*b;}


ll x,y,d;       //d is gcd x and y are coefficients
void exten_euclid(ll a,ll b)
{	ll temp;
	if(b==0)
	{
		d=a;
		x=1;
		y=0;
	}
	else
	{
		exten_euclid(b,a%b);
		temp=x;
		x=y;
		y=temp-(a/b)*y;
	}
return;
}



//     O(sqrtn)

vector<ll> factorize(ll n) {
    vector<ll> res;
    for (ll i = 2; i * i <= n; ++i) {
        while (n % i == 0) {
            res.push_back(i);
            n /= i;
        }
    }
    if (n != 1) {
        res.push_back(n);
    }
    return res;
}

*************FACTORIZE IN 0(LOG(N)) PROVIDED N IS INTEGER**************

int minPrime[100000];

void modified_sieve(int n){
for (int i = 2; i * i <= n; ++i) {
    if (minPrime[i] == 0) {         //If i is prime
        for (int j = i * i; j <= n; j += i) {
            if (minPrime[j] == 0) {
                minPrime[j] = i;
            }
        }
    }
}
for (int i = 2; i <= n; ++i) {
    if (minPrime[i] == 0) {
        minPrime[i] = i;
    }
}
}

vector<int> factorize(int n) {
    vector<int> res;
    while (n != 1) {
        res.push_back(minPrime[n]);
        n /= minPrime[n];
    }
    return res;
}

*************************************************
void sieve(int N) {
        bool isPrime[N+1];
        for(int i = 0; i <= N;++i) {
            isPrime[i] = true;
        }
        isPrime[0] = false;
        isPrime[1] = false;
        for(int i = 2; i * i <= N; ++i) {
             if(isPrime[i] == true) {
                 for(int j = i * i; j <= N ;j += i)
                     isPrime[j] = false;
            }
        }
    }


ll binomialCoeff(ll n, ll k)
{
     ll res = 1;
	if(n<k)
	return 0;

    // Since C(n, k) = C(n, n-k)
    if ( k > n - k )
        k = n - k;

    // Calculate value of
    // [n * (n-1) *---* (n-k+1)] / [k * (k-1) *----* 1]
    for (ll i = 0; i < k; ++i)
    {
        res *= (n - i);
        res /= (i + 1);
    }

    return res;
}


bool dfs_cycle(int root){
vis[root]=1;
bool temp=false;
for(int i=0;i<G[root].size();i++)
{
int v=G[root][i];
if(vis[v]==0)
temp|=dfs_cycle(v);
else if(vis[v]==1)
return true;
}
vis[root]=2;
return temp;
}

vector<vector<int> > G;
vector<int> vis;
bool is_cyclic(int n){
    vector<int> mark(2000,-1);
    queue<int> q;
    q.push(n);
    mark[n]=0;
    while(!q.empty()){
	int v=q.front();
	q.pop();
 	for(int i=0;G[v].size();i++){
	    int nbs=G[v][i];
	    if(mark[nbs]==-1){
		q.push(nbs);
		mark[nbs]=0;
	    }
	    else if(mark[nbs]==0) return true;
	}
 	mark[v]=1;
    }
    return false;
}





void DFS_UTIL(ll u){
	vis[u]=true;
	
	for(auto it=G[u].begin();it!=G[u].end();it++){
		if(vis[*it]==false)
		{
		DFS_UTIL(*it);
		}
	}
}

*/
/*
map<int,int> par,rk;
void make_set(int n) // the rank and parent
{
    for(int i=1; i<=n; i++) { par[i] = i; rk[i] = 1; }
}

int find( int x)   // with PATH COMPRESSION
{
    if (x== par[x]) return x;
    int p = find(par[x]);
    par[x] = p;
    return p;
}

int uni(int x, int y) // with UNION BY rank
{
    int p1 = find(x);
    int p2 = find(y);
    if (p1 == p2 ) return 0;
    if ( rk[p1] > rk[p2] ) { par[p2] = p1; rk[p1] += rk[p2]; }
    else { par[p1] = p2; rk[p2] += rk[p1]; }
     return 1;
}


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


void bfs(int o,int dp[],int n){
	queue<int> q;
	q.push(o);
	dp[o]=0;
	bool visited[n+1]={0};
	visited[o]=true;
	while(q.empty()==false){
		int a=q.front();
		q.pop();
		for (int u : G[a]) {
            if(visited[u]==false){
                visited[u]=true;
                q.push(u);

                dp[u]=dp[a]+1;
            }
        }
	}
}

*/
vector<vector<ll> >G;
vector<bool> vis;
map<ll,vector<ll> > mpx;
map<ll,ll> mpx2;
int main()
{
fifo;
ll n,m,a,b,x;
cin>>n;
vll v;
forn(i,n){
	cin>>x;
	v.pb(x);
	mpx[i+1-x].pb(i+1);
	if(i+1+x == i+1-x)
	mpx2[i+1]=1;
}
ll count=0;
for(int i=0;i<n;i++){
	count+=mpx[i+1+v[i]].size();
	if(mpx2[i+1])
	count--;
	
}
cout<<count<<endl;
return 0;
}


Measured memory_bytes: 51200