#include <bits/stdc++.h>
using namespace std;
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
// using namespace __gnu_pbds;
#define ordered_set tree<pair<int,int>, null_type,less<pair<int,int> >, rb_tree_tag,tree_order_statistics_node_update>
#define standardVar ll n,m,k,num,num1,num2,flag=0
#define nn <<"\n"
#define __ <<" "<<
#define pb(x) push_back(x)
#define mp(x,y) make_pair(x,y)
#define ff first
#define ss second
#define all(v) v.begin(),v.end()
#define ittr(x,v) for(auto x=v.begin();x!=v.end();x++)                  //COMMON FOR LOOP (ITERATOR)
#define itr(x,n) for(int x=0;x<n;x++)                                   //COMMON FOR LOOP (VARIABLE)
#define itrab(x,a,b) for(int x=a;x<b;x++)                               //FOR LOOP WITH SPECIFIED START/END
#define itpr(v) ittr(x,v)cout<<*x<<" "; cout nn                         //PRINTS VECTOR,SET,MAP OF INTS
#define itppr(v) ittr(x,v)cout<<(*x).ff<<" "<<(*x).ss<<"  "; cout nn    //PRINTS VECTOR,SET,MAP OF PAIRS
#define sz(x) (ll)((x).size())

#define pi   3.14159265359
#define dTor(x) ((x)*pi)/180.0
#define INF  1000000000
#define INFF 1000000000000000000

typedef long long int ll;                       //TYPEDEFS
typedef long double ld;
typedef vector<ll> vl;
typedef vector<bool> vb;
typedef map<ll,ll> ml;
typedef set<ll> sl;
typedef string str;
typedef pair<ll, ll> pl;
typedef vector< pl > vpl;
typedef map<pl,ll> mpl;
typedef set< pl > spl;
typedef vector<str> vst;
typedef vector<vl> vvl;
typedef vector<vb> vvb;

const ll M=1000000007;                          //COMMON MODULO PRIMES
const ll N=998244353;

ll power(ll x, ll y, ll p){                     //MODULAR EXPONENTIATION
    ll res=1;
    x=x%p;
    if(x==0)
        return 0;
    while(y>0){
        if(y&1)
            res=(res*x)%p;
        y=y>>1;
        x=(x*x)%p;
    }
    return res;
}
ll inv(ll x, ll m){                             //FERMAT INVERSE
    return power(x,m-2,m);
}
ll nCr(ll n, ll r){                             //nCr MOD P  
    vl fac(10);
    if (r==0) 
        return 1;
    return (fac[n]*inv(fac[r],M)%M*inv(fac[n-r],M)%M)%M;
}
struct comp{                                    //COMPARISON ON PAIRS
    template<typename T>        
    bool operator()(const T &a, const T &b){
        if (a.ff==b.ff)
            return (a.ss<b.ss);
        else
            return (a.ff<b.ff);
    }
};
bool mod(const pair<ll,ll> &a, const pair<ll,ll> &b){   //COMPARATOR FOR PAIRS (SORTING)
    if(a.ff!=b.ff)
        return (a.ff<b.ff);
    else
        return (a.ss>b.ss);
}
void sieve(ll mn, ll mx, vl &primes){                                   //SIEVE OF ERATOSTHENES
    vector<bool> pr(100000+1,true);
    for(ll i=2;i*i<=100000;i++)
        if(pr[i]==1)
            for(ll j=i*i;j<=100000;j+=i)
                pr[j]=0;
    for(int i=2;i<100000;i++)
        if(pr[i])
            primes.pb(i);
}
bool isValid(ll i, ll j, vst &v, ll n, ll m){   //CHECKS FOR OVERFLOW IN MATRIX
    if(i>=1&&i<=n&&j>=1&&j<=m){
        return true;
    }
    return false;
}
void dfs(ll v, vvl &adj, vb &vis){
    vis[v]=true;
    itr(i,sz(adj[v])){
        if(!vis[adj[v][i]]){
            dfs(adj[v][i],adj,vis);
        }
    }
}

//MAIN FUNCTION STARTS HERE

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    standardVar;
    cin>>n;
    vl v(n+1);
    ml a,b;
    itrab(i,1,n+1){
        cin>>v[i];
        a[v[i]+i]++;
        if(i>v[i])
            b[i-v[i]]++;
    }
    ll ans=0;
    ittr(it,a){
        ans+=it->ss*b[it->ff];
    }
    cout<<ans;
    return 0;
}


Measured memory_bytes: 30024