#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <stdlib.h>     /* srand, rand */
#include <time.h>       /* time */
#include <stdio.h>
#include <time.h>
#include <chrono>
#include <ctime>
//#pragma GCC target ("avx2")
//#pragma GCC optimization ("O3")
//#pragma GCC optimization ("unroll-loops")
#define mt make_tuple
#define ll long long
#define ld long double
#define eb emplace_back
#define fi first
#define pb push_back
#define endl '\n'
#define all(x) (x).begin(), (x).end()
#define SZ(x) ((int)((x).size()))
#define rall(x) (x).rbegin(), (x).rend()
#define forn(i, n) for (int i = 0; i < (int)(n); ++i)
#define fore(i, a, b) for (int i = (int)(a); i <= (int)(b); ++i)
#define make_unique(vec) sort(all(vec)); vec.resize(unique(all(vec)) - vec.begin());
#define scanVec(vec) for(int i = 0; i < SZ(vec) ; i++){ cin >> vec[i];}
#define printVec(vec) for(int i = 0; i < SZ(vec) ; i++){ cout<<vec[i]<<" ";}
#define mod(a,b) ((a%b +b)%b)
#define bit(x,i) (x&(1<<i))  //select the bit of position i of x
#define lowbit(x) ((x)&((x)^((x)-1))) //get the lowest bit of x
#define hBit(msb,n) asm("bsrl %1,%0" : "=r"(msb) : "r"(n)) //get the highest bit of x, maybe the fastest
//#define error(args...) { string _s = #args; replace(_s.begin(), _s.end(), ',', ' '); stringstream _ss(_s); istream_iterator<string> _it(_ss); err(_it, args); }
#define IN(i,l,r) (l<i&&i<r)
#define LINR(i,l,r) (l<=i&&i<=r)
#define LIN(i,l,r) (l<=i&&i<r)
#define INR(i,l,r) (l<i&&i<=r)
#define lastEle(vec) vec[vec.size()-1] 
#define FOREACH(i,t) for (typeof(t.begin()) i=t.begin(); i!=t.end(); i++) 
#define ll long long 
#define ull unsigned long long
#define ui unsigned int
#define us unsigned short
#define INF 1001001001
//#define PI 3.1415926535897932384626
 
using namespace std;
 using namespace __gnu_pbds;
 template <typename T>
 using ordered_set =
    tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
 
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<pii> vpi;
typedef vector<vi> vvi;
 
#define deb(args...) { error(args); cout << flush;}
 
#define error(args...) { string _s = #args; replace(_s.begin(), _s.end(), ',',' '); stringstream _ss(_s); istream_iterator<string> _it(_ss); err(_it, args); }
 
void err(istream_iterator<string> it) {}
template<typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
	cerr << " [" << *it <<  ": " << a << "] " << flush;
	err(++it, args...);
}
//----------------------------------------------------------------------------------------------------------------------
 
struct custom_hash {
    static uint64_t splitmix64(uint64_t x) {
        // http://xorshift.di.unimi.it/splitmix64.c
        x += 0x9e3779b97f4a7c15;
        x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
        x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
        return x ^ (x >> 31);
    }
 
    size_t operator()(uint64_t x) const {
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return splitmix64(x + FIXED_RANDOM);
    }
}; 
 
// unordered_map<long long, int, custom_hash> safe_map;
// unordered_set<long long, custom_hash> safe_set;
 
//----------------------------------------------------------------------------------------------------------------------
void swap(ll & a, ll & b){
    auto temp = a;
    a = b;
    b = temp;
}
 
const ll MOD = 1e9 + 7;
 
unsigned ll inf = 1e17;
 
void no(){
    cout <<"NO" << endl;
   //exit(0);
}
 
void yes(){
    cout <<"YES" << endl; //exit(0);
}
 
const ll md = 998244353;
 
const int MAXN = 5*1e3 + 10;
 
 
void sl(){
	int n; cin >> n;
	vector< int > arr (n); scanVec(arr);
	map< int , ll > freq;
	freq[arr[0]] = 1;
	ll ans = 0;
	for(int i = 1;i < n;i++){
		int k = i - arr[i];
		ans += freq[k];
		//deb(i,ans); cout << endl;
		freq[arr[i] + i]++;
	}
	cout << ans << endl;
}
  
int main(){
	//freopen("input.txt","r",stdin);
    //freopen("output.txt","w",stdout);
	ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    //srand(time(NULL));
    // 1 - multiple tests
    // 0 - single test
    int ts = 0;
    if(ts == 1) cin >> ts;
    else ts = 1;
    while(ts--) sl(); 
    return 0;
} 


Measured memory_bytes: 26464