#ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>

#if __cplusplus >= 201103L
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>
#endif

// C++
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>

#if __cplusplus >= 201103L
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#endif




#define int long long
#define rng(i,l,r) for(size_t i=(l);i<(r);++i)
#define rep(i,n) rng(i,0,n)
#define gnr(i,l,r) for(size_t i=(r)-1;i>=(l);i--)
#define per(i,b) gnr(i,0,b)
#define ALL(obj) (obj).begin(), (obj).end()   //1,2,3,...
#define rALL(obj) (obj).rbegin(), (obj).rend()//...,3,2,1

using namespace std;
//using ll = long long;


const int MOD = 1000000007;
const int INF = 1e18;

const int dx4[4] = {1, 0, -1, 0};
const int dy4[4] = {0, 1, 0, -1};
const int dx8[8] = {1,1,0,-1,-1,-1,0,1};
const int dy8[8] = {0,1,1,1,0,-1,-1,-1};

const double pi = acos(-1);



template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }


using Graph = vector<vector<int>>;
int gcd(int a,int b){return b?gcd(b,a%b):a;}
//*max_element(ALL(vector))
//count(ALL(vector),num) -> n



void solve(){
    
    // remove the bottom 3 lines when you submit this code.
    std::ifstream in("sample.txt");
    std::cin.rdbuf(in.rdbuf());

    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int n; cin >> n;
    vector<int> A(n);
    for (int i = 0; i < n; i++) cin >> A[i];
    
    map<int,int> mapsum;
    map<int,int> mapdel;
    
    for (int i = 0; i < n; i++) {
        int sum = i+1 + A[i];
        int del = i+1 - A[i];
        
        mapsum[sum]++;
        mapdel[del]++;
        
    }
    
    int ans = 0;
    for (auto  itr = mapsum.begin(); itr != mapsum.end(); itr++) {
        int now = itr->first;
        int sumnum = itr->second;
        
        ans += mapdel[now]*sumnum;
    }
    
    
    cout << ans << endl;
    
    
    
    
}





signed main() {
    solve();
    return 0;
}



Measured memory_bytes: 38908