Problem:
Score: 500 points

Problem Statement

You are the top spy of AtCoder Kingdom. To prevent the stolen secret from being handed to AlDebaran Kingdom, you have sneaked into the party where the transaction happens.

There are N attendees in the party, and they are given attendee numbers from 1 through N. The height of Attendee i is A_i.

According to an examination beforehand, you know that a pair of attendees satisfying the condition below will make the transaction.

The absolute difference of their attendee numbers is equal to the sum of their heights.

There are \frac{N(N-1)}{2} ways to choose two from the N attendees and make a pair. Among them, how many satisfy the condition above?

P.S.: We cannot let you know the secret.

Constraints

All values in input are integers.

2 \leq N \leq 2 \times 10^5

1 \leq A_i \leq 10^9\ (1 \leq i \leq N)

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 \dots A_N

Output

Print the number of pairs satisfying the condition.

Sample Input 1

6
2 3 3 1 3 1

Sample Output 1

3

A_1 + A_4 = 3, so the pair of Attendee 1 and 4 satisfy the condition.

A_2 + A_6 = 4, so the pair of Attendee 2 and 6 satisfy the condition.

A_4 + A_6 = 2, so the pair of Attendee 4 and 6 satisfy the condition.

No other pair satisfies the condition, so you should print 3.

Sample Input 2

6
5 2 4 2 8 8

Sample Output 2

0

No pair satisfies the condition, so you should print 0.

Sample Input 3

32
3 1 4 1 5 9 2 6 5 3 5 8 9 7 9 3 2 3 8 4 6 2 6 4 3 3 8 3 2 7 9 5

Sample Output 3

22

Solution in Java (OpenJDK 11.0.6) (judge verdict: Time Limit Exceeded)

import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.io.Serializable;
import java.math.BigInteger;
import java.util.*;
 
 
public class Main {
	
	
	static int[] H;
	static boolean[] seen;

	public static void main(String[] args) throws Exception {
		
		
		//int N = scan.nextInt();
		//long L = scan.nextLong();
		//char[] A = scan.next().toCharArray();
		long ans = 0;
		int N = scan.nextInt();
		int[] A = new int[N];
		int[][] B = new int[N][2];
		for(int i=0; i<N; i++){
			A[i] = scan.nextInt();
			B[i][0] = i;
			B[i][1] = i + A[i];
		}
		Arrays.sort(B, (a,b) -> Integer.compare(a[1], b[1]));
		for(int i=N-1; i>=0; i--){
			int K = i-A[i];
			int lb = -1;
			int ub = N;
			while(ub-lb > 1){
				int mid = (lb + ub) / 2;
				if(B[mid][1] >= K){
					ub = mid;
				}else{
					lb = mid;
				}
			}
			while(B[ub][1] == K){
				if(B[ub][0]<i)ans++;
				ub++;
			}
		}
		
		out.println(ans);

		
				
		out.flush();
	}
	static boolean dfs(List<ArrayList<Integer>> G,int root, int from, boolean[] seen ){
		seen[from] = true;
		for(int to : G.get(from)){
			if(seen[to])continue;
			if(H[root] <= H[to])return false;
			
			if(!dfs(G,root, to,seen))return false;
		}
		return true;
	}

	static FastScanner scan = new FastScanner();
	static PrintWriter out = new PrintWriter(System.out);			 
	static long MOD = 1_000_000_007;
	static long INF = 1_000_000_007;
	//二項係数関連ここから-----
	static boolean isPreprocessing = false;	//二項係数を計算するための前処理を行っているかどうかのフラグ
	static int MAX = 666667;				//MAXまで階乗やそれらの逆元のテーブルを保持する。
	static long[] fac;	//fac[i] := i! % MOD
	static long[] inv;	//inv[i] := mod. MODにおける i の逆元
	static long[] finv;	//finv[i] := mod. MODにおける i! の逆元(invの累積"積")
	//二項係数関連おわり------
	
	static int[] roots;		//Union-find木用
	//static List<ArrayList<Integer>> G;	//グラフ用

	static List<List<E>> G = new ArrayList<>();	//グラフ用
	static List<E> Edges = new ArrayList<>();	//グラフ用
	static int V;								//グラフ用頂点の数
	static int E;								//グラフ用辺の数
	static long[] X = new long[1];				//拡張ユークリッドの互除法用
	static long[] Y = new long[1];				//拡張ユークリッドの互除法用
	//旧入力
	//Scanner sc = new Scanner(System.in);
	//int N = Integer.parseInt(sc.next());
	static int[] dx = {1,-1,0,0};
	static int[] dy = {0,0,1,-1};
	static int[] dx8 = {1,-1,0,0,1,1,-1,-1};
	static int[] dy8 = {0,0,1,-1,1,-1,1,-1};
	
	
	/*	グラフ形式の入力を行う
	 * 	前提：M個の辺があるとき、以下の形式で辺情報が与えられることを期待している
	 * 		1 2
	 * 		2 3
	 *  	2 4
	 *  入力：
	 *  	int		N		: 頂点の数
	 *  	int 	M		: 辺の数
	 *  	boolean	isArrow	: 無向グラフか有向グラフかを表すフラグ（trueならば有向グラフ）
	 *  出力：ArrayList<ArrayList<Integer>>
	 * 
	 */
	static ArrayList<ArrayList<Integer>> inputG(int N, int M, boolean isArrow){
		ArrayList<ArrayList<Integer>> G = new ArrayList<>();
		
		for(int i=0; i<N; i++){
			ArrayList<Integer> V = new ArrayList<>();
			G.add(V);
		}
		for(int i=0; i<M; i++){
			int u = scan.nextInt() - 1;
			int v = scan.nextInt() - 1;
			G.get(u).add(v);
			if(!isArrow)G.get(v).add(u);
		}
		return G;
	}


	//a_i≧kとなるような最小のiを求める(0≦i≦N-1)
	//存在しなかったら-1を返す
	static int lower_bound(long[] A, long k){
		int N = A.length;
		int lb = -1;
		int ub = N;
		while(ub-lb > 1){
			int mid = (lb + ub) / 2;
			if(A[mid] >= k){
				ub = mid;
			}else{
				lb = mid;
			}
		}
		return ub;
	}

	//k≧a_iとなるような最大のiを求める(0≦i≦N-1)
	//存在しなかったらa_N-1を返す
	static int upper_bound(long[] A, long k){
		int N = A.length;
		int lb = -1;
		int ub = N;
		while(ub-lb > 1){
			int mid = (lb + ub) / 2;
			if(A[mid] <= k){
				lb = mid;
			}else{
				ub = mid;
			}
		}
		if(lb == N) lb = N-1;
		return lb;
	}
	
	//aの逆元a^-1を求める
	//条件：aとmodが互いに素であること(aがmodの倍数でないこと)
	static long modinv(long a, long mod){
		long b = mod;
		long u = 1;
		long v = 0;
		while(b>0){
			long t = a/b;
			a -= t*b;
			long tmp = b;
			b = a;
			a = tmp;
			u -= t*v;
			tmp = v;
			v = u;
			u = tmp;
		}
		u %= mod;
		if(u<0) u+=mod;
		return u;
	}
	
	//繰り返し二乗法(x^n)
	static long pow(long x, long n){
		long sum = 1;
		while(n>0){
			if((n&1) == 1 ){
				sum = sum * x;
			}
			x = x * x;
			n >>= 1;
		}
		return sum;
	}
	
	//繰り返し二乗法(x^n % mod)
	static long modPow(long x, long n, long mod){
		long sum = 1;
		while(n>0){
			if((n&1) == 1 ){
				sum = sum * x % mod;
			}
			x = (x * x) % mod;
			n >>= 1;
		}
		return sum;
	}
	
	//オイラーのφ関数（nに対して、nと互いに素である1以上n以下の自然数の個数）
	static int phi(int n){
		List<long[]> primes = factorization(n);//素因数分解
		int rtn = n;
		for(long[] prime : primes){
			int p = (int)prime[0];
			rtn /= p;
			rtn *= p-1;
		}
		return rtn;
	}
	
	//エラトステネスの篩
	//要素数がn+1のboolean配列を返す。
	//素数のindexはtrueとなり、それ以外はfalseとなる
	static boolean[] eratosthenes(int n){
		boolean[] flg = new boolean[n+1];
		Arrays.fill(flg, true);
		flg[0] = flg[1] = false;
		for(int i=2; i<n+1; i++){
			if(!flg[i])continue;
			for(int j=i*2; j<n+1; j += i)flg[j] = false;
		}
		return flg;
	}
	
	//素数判定
	static boolean isPrime(int n){
		int tmp = 2;
		boolean flg = true;
		while(tmp*tmp<=n){
			if (n%tmp == 0)flg = false;
			tmp++;
		}
		if(n <= 1)flg = false;
		return flg;
	}
	
	//拡張ユークリッドの互除法
	//返り値、aとbの最大公約数
	//X[0]とY[0]が、ax+by=gcd(a,b)を満たすようになる
	static long extGCD(long a, long b, long[] X, long[] Y){
		if(b == 0){
			X[0] = 1;
			Y[0] = 0;
			return a;
		}
		long d = extGCD(b, a%b, Y, X);
		Y[0] -= a/b * X[0];
		
		return d;
	}
	
	//２点間の距離
	public static double distance(double X1, double Y1, double X2, double Y2){
		return Math.sqrt((X1-X2)*(X1-X2) + (Y1-Y2)*(Y1-Y2));
	}
	
	//階乗
	public static int fact(int a){
		if(a == 0 || a == 1)return 1;
		return a*fact(a-1);
	}
		
	//約数の配列を返す
	static long[] divisor(long num){
		Set<Long> divisors = new TreeSet<>();
		long tmp=1;
		while(tmp*tmp<=num){
			if(num%tmp == 0){
				divisors.add(tmp);
				divisors.add(num/tmp);
			}
			tmp++;
		}
		Long[] rtn = new Long[divisors.size()];
		divisors.toArray(rtn);
		long[] ans = new long[rtn.length];
		for(int i=0; i<rtn.length; i++){
			ans[i] = rtn[i];
		}
		return ans;
	}
	
	//GCD最大公約数を返す
	static long gcd(long a, long b){
		if(b == 0) return a;
		return gcd(b, a%b);
	}

	static BigInteger biggcd(BigInteger a, BigInteger b){
		if(b == BigInteger.ZERO) return a;
		return biggcd(b, a.remainder(b));
	}
	
	//Bellman-Ford法
	//辺情報とstartの頂点を指定すると、すべての頂点への最短コストを返す（負の重みの辺もOK）
	//(問題に応じて要変化)1→Vのパスに負の閉路がある場合はindex:Vの要素に-1を入れて返す。
	static long[] bellmanFord(List<E> Edges, int from){
		long[] rtn = new long[V+1];
		Arrays.fill(rtn, INF);
		rtn[from] = 0;
		
		for(int i=0; i<V-1; i++){
			boolean update = false;
			for(E e : Edges){
				if(rtn[e.from] != INF && rtn[e.to] > rtn[e.from] + e.weight){
					rtn[e.to] = rtn[e.from] + e.weight;
					update = true;
				}
			}
			if(!update)break;
		}	
		//閉経路判定
		boolean[] is_update = new boolean[V];
		Arrays.fill(is_update, false);
		for(int i=0; i<V; i++){
			for(E e : Edges){
				if(rtn[e.from] != INF && rtn[e.to] > rtn[e.from] + e.weight){
					rtn[e.to] = rtn[e.from] + e.weight;
					is_update[e.to] = true;
				}
			}
		}
		
		if(is_update[V-1]){
			rtn[V] = -1;
		}
		return rtn;
	}
	
	//Kruskal法
	//最小全域木を構成したときのコストの総和を返す
	static long kruskal(List<E> Edges){
		Collections.sort(Edges);
		UnionFind uni = new UnionFind(V);
		long res = 0L;
		for (E e : Edges){
			if(!uni.same(e.from, e.to)){
				uni.join(e.from, e.to);
				res += e.weight;
			}
		}
		return res;
	}
	
	//Dijkstra法
	//辺情報とstartの頂点を指定すると、すべての頂点への最短コストを返す
	static long[] dijkstra(List<List<E>> G, int from){
		int N = G.size();
		long[] rtn = new long[N];
		Arrays.fill(rtn, INF);
		rtn[from] = 0;
		Queue<V> que = new PriorityQueue<>();
		
		que.add(new V(from,0));
        while(!que.isEmpty()){
            V doneNode = que.poll();
            for(E e: G.get(doneNode.num)){
                  long alt = rtn[doneNode.num] + e.weight;
                  if(rtn[e.to] > alt){
                        rtn[e.to] = alt;
                        que.add(new V(e.to, alt));
                  }     
            }
        }
		return rtn;
	}
	
	//グラフ用：辺
	static class E implements Comparable<E>{
		int from;
		int to;
		long weight;
		public E(int from, int to, long weight){
			this.from = from;
			this.to = to;
			this.weight = weight;
		}
		@Override
		public int compareTo(E o) {
			// TODO Auto-generated method stub
			return Long.compare(this.weight, o.weight);
		}
	}
	//グラフ用：頂点
	static class V implements Comparable<V>{
		int num;
		long cost;
		public V(int num, long cost){
			this.num = num;
			this.cost = cost;
		}
		//コストの低い順に並べる(PriorityQueue用)
		@Override
		public int compareTo(V o) {
			return Long.compare(this.cost, o.cost);
		}
	}
	
	//Union-Find木
	static class UnionFind{
		private int[] roots;
		public UnionFind(int N){
			this.roots = new int[N];
			for(int i=0; i<N; i++){
				this.roots[i] = i;
			}
		}
		
		public boolean same(int a, int b){
			if(root(a) == root(b)){
				return true;
			}else{
				return false;
			}
		}
		
		public int root(int a){
			if(a == this.roots[a])return a;
			return this.roots[a] = root(this.roots[a]);
		}
		
		public void join(int a, int b){
			if(root(a) == root(b))return;
			this.roots[root(b)] = root(a);
		}
		
		public int[] getRoots(){
			return this.roots;
		}
	}
 
	//素因数分解の結果AとBに対して、A*Bの素因数分解の結果を返す
	static List<long[]> factorizationMerge(List<long[]> A, List<long[]> B){
		List<long[]> primes = new ArrayList<>(A);
		for(long[] b : B){
			boolean isExist = false;
			for(long[] a : primes){
				if(a[0] == b[0]){
					isExist = true;
					a[1] += b[1]; 
				}
			}
			if(!isExist)primes.add(b);
		}
		return primes;
	}
	
	//素因数分解をする(index_0:素数 index_1:個数)
	static List<long[]> factorization(long M){
		List<long[]> primes = new ArrayList<>();
 
		long now_prime = 2;
		long now_M = M;
		while(now_prime*now_prime<=M){
			if(now_M%now_prime == 0){
				now_M /= now_prime;
				long[] prime = new long[2];
				prime[0] = now_prime;
				prime[1] = 1;
				while(now_M%now_prime == 0){
					now_M /= now_prime;
					prime[1]++;
				}
				primes.add(prime);
				
			}
			if(now_prime%2 == 0){
				now_prime++;
			}else{
				now_prime += 2;
			}
		}
		if (now_M>1){
			long[] prime = new long[2];
			prime[0] = now_M;
			prime[1] = 1;
			primes.add(prime);
		}
		
		return primes;
	}
	
	//２つのint配列を連結させる
	static int[] joinArray(int[] A, int[] B){
		int[] tmp = new int[A.length + B.length];
		System.arraycopy(A, 0, tmp, 0, A.length);
		System.arraycopy(B, 0, tmp, A.length, B.length);
		return tmp;
	}
	
	//２つのint配列が等しいかどうか判定する
	static boolean arrayEqual(int[] A, int[] B){
		if(A.length != B.length)return false;
		for(int i=0; i<A.length; i++){
			if(A[i] != B[i])return false;
		}
		return true;
	}
	
	//int配列をindex順に見て、要素の重複があった場合その要素と間の要素を削除する12123→23
	static int[] calc(int[] A){
		boolean[] tmp = new boolean[A.length];
		Arrays.fill(tmp, true);
		int fetch = 0;
		while(fetch<A.length-1){
			boolean isExist = false;
			for(int i=fetch+1; i<A.length;i++){
				if(A[fetch] == A[i]){
					for(int j=fetch;j<=i;j++){
						tmp[j]=false;
					}
					fetch = i+1;
					isExist = true;
					break;
				}
			}
			if(!isExist)fetch++;
		}
		List<Integer> list = new ArrayList<>();
		for(int i=0; i<A.length; i++){
			if(tmp[i])list.add(A[i]);
		}
		
		int[] foo = new int[list.size()];
		for(int i=0; i<list.size(); i++){
			foo[i] = list.get(i);
		}
		/*
		for(boolean t : tmp)
			System.out.print(t+" ");
		*/
		return foo;
	}
		
	//aとbの最小公倍数を返す
	static long lcm (long a, long b) {
		return ((a/gcd(a,b))*b);
	}
	
	//二項係数用の前処理
	static void COMinit(){
		//二項係数演算用前処理
		fac = new long[MAX+1];
		fac[0] = 1L;
		fac[1] = 1L;
		inv = new long[MAX+1];
		inv[1] = 1L;
		finv = new long[MAX+1];
		finv[0] = 1L;
		finv[1] = 1L;
		for(int i=2;i<MAX;i++){
			fac[i] = fac[i-1] * i % MOD;
			inv[i] = MOD - inv[(int)(MOD%i)] * (MOD/i) % MOD;
			finv[i] = finv[i-1] * inv[i] % MOD;
		}
		isPreprocessing = true;
	}
	
	//二項係数i_C_jをMODで割った余りを返す
	static long COM(int i, int j){
		if(i<j)return 0;
		if(i<0 || j<0)return 0;
		if(!isPreprocessing)COMinit();
		return fac[i] * (finv[j] * finv[i-j] % MOD) % MOD;
	}
	
	//入力
	//https://qiita.com/p_shiki37/items/a0f6aac33bf60f5f65e4
	static class FastScanner {
	    private final InputStream in = System.in;
	    private final byte[] buffer = new byte[1024];
	    private int ptr = 0;
	    private int buflen = 0;
	    private boolean hasNextByte() {
	        if (ptr < buflen) {
	            return true;
	        }else{
	            ptr = 0;
	            try {
	                buflen = in.read(buffer);
	            } catch (IOException e) {
	                e.printStackTrace();
	            }
	            if (buflen <= 0) {
	                return false;
	            }
	        }
	        return true;
	    }
	    private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
	    private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
	    public boolean hasNext() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; return hasNextByte();}
	    public String next() {
	        if (!hasNext()) throw new NoSuchElementException();
	        StringBuilder sb = new StringBuilder();
	        int b = readByte();
	        while(isPrintableChar(b)) {
	            sb.appendCodePoint(b);
	            b = readByte();
	        }
	        return sb.toString();
	    }
	    public long nextLong() {
	        if (!hasNext()) throw new NoSuchElementException();
	        long n = 0;
	        boolean minus = false;
	        int b = readByte();
	        if (b == '-') {
	            minus = true;
	            b = readByte();
	        }
	        if (b < '0' || '9' < b) {
	            throw new NumberFormatException();
	        }
	        while(true){
	            if ('0' <= b && b <= '9') {
	                n *= 10;
	                n += b - '0';
	            }else if(b == -1 || !isPrintableChar(b)){
	                return minus ? -n : n;
	            }else{
	                throw new NumberFormatException();
	            }
	            b = readByte();
	        }
	    }
	    public int nextInt() {
	        long nl = nextLong();
	        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
	        return (int) nl;
	    }
	    public double nextDouble() { return Double.parseDouble(next());}
	}
}