#include <bits/stdc++.h>
using namespace std;
#define endl '\n'
#define rep(i,a,n) for (int i=a;i<n;i++)
#define per(i,a,n) for (int i=n-1;i>=a;i--)
#define pb push_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define SZ(x) ((int)(x).size())
typedef vector<int> VI;
typedef long long ll;
typedef pair<int,int> PII;
typedef double db;
mt19937 mrand(random_device{}()); 
const ll mod=1000000007;
int rnd(int x) { return mrand() % x;}
ll powmod(ll a,ll b) {ll res=1;a%=mod; assert(b>=0); for(;b;b>>=1){if(b&1)res=res*a%mod;a=a*a%mod;}return res%mod;}
ll gcd(ll a,ll b) { return b?gcd(b,a%b):a;}
ll lcm(ll a,ll b) { return a/gcd(a,b)*b;}
const std::vector<std::pair<int, int>> moves{{0, -1}, {-1, 0}, {0, 1}, {1, 0}};
inline int bitCount(int x) {return __builtin_popcount(x);}// 1001 -> 2
inline int bitCount(ll x) {return __builtin_popcountll(x);} 
inline int highestOneBit(int x) {return 1 << (31 - __builtin_clz(x | 1));} // 1001 -> 8
inline int binaryDigits(int x) { return 32 - __builtin_clz(x | 1);} // 1001 -> 4
// head
template<typename A>string to_string(A* ptr){stringstream ss; ss << "0x" << std::setw(16) << std::setfill ('0') << std::hex << (uint64_t)(uintptr_t)ptr;return ss.str();}
template <typename A, typename B>string to_string(pair<A, B> p);
template <typename A, typename B, typename C>string to_string(tuple<A, B, C> p);
template <typename A, typename B, typename C, typename D>string to_string(tuple<A, B, C, D> p);
string to_string(const string& s){return '"' + s + '"';}
string to_string(const char* s){return to_string((string) s);}
string to_string(bool b) {return (b ? "true" : "false");}
string to_string(vector<bool> v) {bool first = true;string res = "{";for (int i = 0; i < static_cast<int>(v.size()); i++) {if (!first) {res += ", ";}first = false;res += to_string(v[i]);}res += "}";return res;}
template <size_t N>string to_string(bitset<N> v) {string res = "";for (size_t i = 0; i < N; i++) {res += static_cast<char>('0' + v[i]);}return res;}
template <typename A>string to_string(A v) {bool first = true;string res = "{";for (const auto &x : v) {if (!first) {res += ", ";}first = false;res += to_string(x);}res += "}";return res;}
template <typename A, typename B>string to_string(pair<A, B> p) {return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";}
template <typename A, typename B, typename C>string to_string(tuple<A, B, C> p) {return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";}
template <typename A, typename B, typename C, typename D>string to_string(tuple<A, B, C, D> p) {return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";}
void debug_out() { cerr << endl; }
template <typename Head, typename... Tail>void debug_out(Head H, Tail... T) {cerr << " " << to_string(H);debug_out(T...);}
#if !ONLINE_JUDGE && 1
#define dbg(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define dbg(...) 42
#endif
// Debug

int main() {
	ios::sync_with_stdio(0); 
	cin.tie(0); cout.tie(0);
	int n;
	cin >> n;
	VI h(n);                                            
	rep(i, 0, n) cin >> h[i];
	map<int, int> s;
	ll cnt = 0;
	rep(j, 0, n){
		cnt += s[h[j]-j];
		s[-h[j]-j]++;
	}
	cout << cnt << endl;
}




Measured memory_bytes: 20992