#include<bits/stdc++.h>
    #include <ext/pb_ds/tree_policy.hpp> 
    #include <functional> // for less 
    using namespace __gnu_pbds; 
    using namespace std;
    //typedef tree<ll, null_type, less<ll>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;    
    #define ll            long long
    #define pb            push_back
    #define ppb           pop_back
    #define si            set <ll>
    #define endl          '\n'
    #define fr            first
    #define sc            second
    #define mii           map<ll,ll>
    #define msi           map<string,ll>
    #define mis           map<ll,string>
    #define rep(i,a,b)    for(ll  i=a;i<b;i++)
    #define all(v)        v.begin(),v.end()
    #define sort(v)       sort(all(v))
    #define pii           pair<ll ,ll >
    #define vi            vector<ll >
    #define vii           vector<pair<ll,ll>>
    #define vs            vector<string>
    #define sz(x)         (ll)x.size()
    #define M             1000000007
    #define bs            binary_search
    #define rev(a)        reverse(all(a));
    #define sp(n)         setprecision(n)
    #define spl           " "
    #define arr(a,n)      rep(i,0,n) cin>>a[i]
    #define mod           998244353
    #define print_vector(a)  for(auto t:a) cout<<t<<" ";cout<<endl;
    #define print_arr(a,n)   rep(i,0,n) cout<<a[i]<<" "; cout<<endl;        
    #define show(a,n)     for(ll i=0;i<n;i++) cout<<a[i]<<" "; cout<<endl;
    #define debug(x)      cout << #x << " is " << x << endl;
    #define time          cout << "\nTime elapsed: " << 1000 * clock() / CLOCKS_PER_SEC << "ms\n";
    #define INF           1ll<<31
    #define hi            cout<<"hi"<<endl;
    const ll maxn=int(3e5)+500;
    const ll maxa=(1<<20)+3;
    ll bpow(ll a, ll b, ll mm = M)
    {
        ll res = 1;
     
        while(b)
        {
            if(b & 1)
                res = (res * a) % mm;
            a = (a * a) % mm;
     
            b >>= 1;
        }
     
        return res;
    }
    vector<ll> ret;
    vector<ll> getFactorization(ll n)
    {
        while (n % 2 == 0)  
        {  
            ret.pb(2);
            n = n/2;  
        }  

        for (ll i = 3; i <= sqrt(n); i = i + 2)  
        { 
            while (n % i == 0)  
            {  
                ret.pb(i); 
                n = n/i;  
            }  
        }  

        if (n > 2)  
           ret.pb(n);
        return ret;  
    }  
     
    ll modInverse(ll A,ll mm)
    {
    return bpow(A,mm-2,mm);
    }
    ll nCrModPFermat(ll n, ll r, ll p) 
    { 
     if (r==0) 
        return 1;
      ll fac[n+1]; 
      fac[0] = 1; 
      for (ll i=1 ; i<=n; i++) 
          fac[i] = fac[i-1]*i%p; 

      return (fac[n]*modInverse(fac[r],p)%p*modInverse(fac[n-r], p)%p)%p; 
    } 
    const ll N=1e5+1e4;
    void solve()
    {
     ll n;
     cin>>n;
     std::vector<ll> a(n);
     std::map<ll,ll> mp;
     rep(i,0,n) cin>>a[i];
     ll ans=0;
     rep(i,0,n)
     {
        ans+=mp[i-a[i]];
        mp[i+a[i]]++;
     }
     
     cout<<ans;

    } 
    signed main()
    {
        ios_base::sync_with_stdio(false);
        cin.tie(0);
        cout.tie(0);
        #ifndef ONLINE_JUDGE
       // freopen("input.txt","r",stdin);
       // freopen("output.txt","w",stdout);
        #endif
        ll t=1;
        //cin>>t;
        while(t--) solve();
    }

    

Measured memory_bytes: 27372