/*************************************************************************
    > File Name: solve.cpp
    > Author: liupo
    > Mail: lanzongwei@gmail.com
    > Created Time: 2020-05-03 21:01:27
 ************************************************************************/

#define GOODOJ
#define SYNC 0

#ifdef GOODOJ
#include <bits/stdc++.h>
#include <ext/pb_ds/priority_queue.hpp>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/rope>
#include <chrono>
#include <random>
using namespace __gnu_pbds;
using namespace __gnu_cxx;
#else
#include <iostream>
#include <cstdio>
#include <cmath>
#include <set>
#include <algorithm>
#include <cstring>
#include <string>
#include <map>
#include <deque>
#include <vector>
#include <limits>
#include <cassert>
#include <sstream>
#include <iterator>
#include <functional>
#endif
using namespace std;

struct InputOutputStream {
    enum { SIZE = 1000001 };
    char ibuf[SIZE], *s, *t, obuf[SIZE], *oh;
    bool eof;

    InputOutputStream() : s(), t(), oh(obuf), eof(false) {}
    ~InputOutputStream() { fwrite(obuf, 1, oh - obuf, stdout); }

    inline char read() {
        if (s == t) t = (s = ibuf) + fread(ibuf, 1, SIZE, stdin);
        return s == t ? -1 : *s++;
    }

    template <typename T>
    inline InputOutputStream &operator>>(T &x) {
        static char c;
        static bool iosig;
        for (c = read(), iosig = false; !isdigit(c); c = read()) {
            if (c == -1) {eof = true; return *this;}
            iosig |= c == '-';
        }
        for (x = 0; isdigit(c); c = read()) x = x * 10 + (c ^ '0');
        if (iosig) x = -x;
        return *this;
    }

    inline void print(char c) {
        if (oh == obuf + SIZE) {
            fwrite(obuf, 1, SIZE, stdout);
            oh = obuf;
        }
        *oh++ = c;
    }

    template <typename T>
    inline void print(T x) {
        static int buf[23], cnt;
        if (x != 0) {
            if (x < 0) print('-'), x = -x;
            for (cnt = 0; x; x /= 10) buf[++cnt] = x % 10 | 48;
            while (cnt) print((char)buf[cnt--]);
        } else print('0');
    }

    template <typename T>
    inline InputOutputStream &operator<<(const T &x) {
        print(x);
        return *this;
    }

	inline void print(const char* x) {
		for(; *x; x++)
			print(*x);
	}
} io;

//#define cin io
//#define cout io

#define endl '\n'
#define fep(i,b,e) for(int i=(b);i<(e);++i)
#define rep(i,x) for(int i=0;i<(x);++i)
#define rap(i,x) for(auto& i : (x))
#define seg(t) (t).begin(), (t).end()
#define ep emplace_back
#define mkp make_pair
#define mkt make_tuple
#define qxx(i,x) for(int i = head[x]; ~i; i = node[i].nex)
#define F first
#define S second
#define lowbit(x) ((-(x))&(x))
#define RE register
#define getchar() getchar_unlocked()
#ifdef DEBUG
void err(istream_iterator<string>){}
template<typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
    cerr << *it << " = " << a << ' ';
    err(++it, args...);
}
#define debug(args...) {string _s=#args;replace(seg(_s),',',' ');\
cerr<<"DEBUG:";istringstream it(_s);\
err(istream_iterator<string>(it), args);cerr<<endl;}
#else
#define debug(...)
#endif

template<typename T> inline bool cmax(T& a,const T& b) {return a<b?a=b,1:0;}
template<typename T> inline bool cmin(T& a,const T& b) {return a>b?a=b,1:0;}

#ifdef GOODOJ
mt19937 rng((uint32_t)chrono::steady_clock::now().time_since_epoch().count());
typedef __gnu_pbds::priority_queue<int> pq;
#endif
typedef std::string str;
typedef long long ll;
typedef double db;
typedef pair<int, int> pa;

const double P = acos(-1.0), eps = 1e-9;
struct point { db x ,y;};
inline int sign(db a) {return a < -eps ? -1 : a > eps;}
#define dot(p1,p2,p3) ((p2.x-p1.x)*(p3.x-p1.x)+(p2.y-p1.y)*(p3.y-p1.y))
#define cross(p1,p2,p3) ((p2.x-p1.x)*(p3.y-p1.y)-(p3.x-p1.x)*(p2.y-p1.y))
#define crossOp(p1,p2,p3) sign(cross(p1,p2,p3))

const int Ma = 2e5 + 100, inf = 0x3f3f3f3f, mod = 1e9 + 7;

int mz[Ma];

map<ll, ll> a, b;

signed main() {
    #if SYNC==1
    ios::sync_with_stdio(false);
    cin.tie(0);
    #endif
	int n; cin >> n;
	rep (i, n) cin >> mz[i];
	rep (i, n) a[mz[i] + i]++, b[i - mz[i]]++;
	ll ans = 0;
	rap (i, a) if (b.count(i.F)) ans += i.S * b[i.F];

	cout << ans << endl;

    return 0;
}


Measured memory_bytes: 27356