using System;
using System.Linq;
using CompLib.Collections;
using CompLib.Util;

public class Program
{
    int N;
    long[] A;
    public void Solve()
    {
        var sc = new Scanner();
        N = sc.NextInt();
        A = sc.LongArray();

        var ar = new long[N];
        for (int i = 0; i < N; i++)
        {
            ar[i] = A[i] - i;
        }

        Array.Sort(ar);
        long ans = 0;
        for (int i = 0; i < N; i++)
        {
            // -A[i]-iを探す
            long target = -A[i] - i;
            int low;
            {
                int ok = A.Length;
                int ng = -1;
                while (ok - ng > 1)
                {
                    int mid = (ok + ng) / 2;
                    if (ar[mid] >= target) ok = mid;
                    else ng = mid;
                }
                low = ok;
            }
            if (low == A.Length || ar[low] != target) continue;
            int high;
            {
                int ok = A.Length;
                int ng = -1;
                while (ok - ng > 1)
                {
                    int mid = (ok + ng) / 2;
                    if (ar[mid] > target) ok = mid;
                    else ng = mid;
                }
                high = ok;
            }
            ans += high - low;
        }
        Console.WriteLine(ans);
    }

    public static void Main(string[] args) => new Program().Solve();
}

// https://bitbucket.org/camypaper/complib
namespace CompLib.Collections
{
    using System;
    using System.Collections.Generic;
    #region Set
    public class Set<T>
    {
        Node root;
        readonly IComparer<T> comparer;
        readonly Node nil;
        public bool IsMultiSet { get; set; }
        public Set(IComparer<T> comparer)
        {
            nil = new Node(default(T));
            root = nil;
            this.comparer = comparer;
        }
        public Set(Comparison<T> comaprison) : this(Comparer<T>.Create(comaprison)) { }
        public Set() : this(Comparer<T>.Default) { }
        public bool Add(T v)
        {
            return insert(ref root, v);
        }
        public bool Remove(T v)
        {
            return remove(ref root, v);
        }
        public T this[int index] { get { return find(root, index); } }
        public int Count { get { return root.Count; } }
        public void RemoveAt(int k)
        {
            if (k < 0 || k >= root.Count) throw new ArgumentOutOfRangeException();
            removeAt(ref root, k);
        }
        public T[] Items
        {
            get
            {
                var ret = new T[root.Count];
                var k = 0;
                walk(root, ret, ref k);
                return ret;
            }
        }
        void walk(Node t, T[] a, ref int k)
        {
            if (t.Count == 0) return;
            walk(t.lst, a, ref k);
            a[k++] = t.Key;
            walk(t.rst, a, ref k);
        }

        bool insert(ref Node t, T key)
        {
            if (t.Count == 0) { t = new Node(key); t.lst = t.rst = nil; t.Update(); return true; }
            var cmp = comparer.Compare(t.Key, key);
            bool res;
            if (cmp > 0)
                res = insert(ref t.lst, key);
            else if (cmp == 0)
            {
                if (IsMultiSet) res = insert(ref t.lst, key);
                else return false;
            }
            else res = insert(ref t.rst, key);
            balance(ref t);
            return res;
        }
        bool remove(ref Node t, T key)
        {
            if (t.Count == 0) return false;
            var cmp = comparer.Compare(key, t.Key);
            bool ret;
            if (cmp < 0) ret = remove(ref t.lst, key);
            else if (cmp > 0) ret = remove(ref t.rst, key);
            else
            {
                ret = true;
                var k = t.lst.Count;
                if (k == 0) { t = t.rst; return true; }
                if (t.rst.Count == 0) { t = t.lst; return true; }


                t.Key = find(t.lst, k - 1);
                removeAt(ref t.lst, k - 1);
            }
            balance(ref t);
            return ret;
        }
        void removeAt(ref Node t, int k)
        {
            var cnt = t.lst.Count;
            if (cnt < k) removeAt(ref t.rst, k - cnt - 1);
            else if (cnt > k) removeAt(ref t.lst, k);
            else
            {
                if (cnt == 0) { t = t.rst; return; }
                if (t.rst.Count == 0) { t = t.lst; return; }

                t.Key = find(t.lst, k - 1);
                removeAt(ref t.lst, k - 1);
            }
            balance(ref t);
        }
        void balance(ref Node t)
        {
            var balance = t.lst.Height - t.rst.Height;
            if (balance == -2)
            {
                if (t.rst.lst.Height - t.rst.rst.Height > 0) { rotR(ref t.rst); }
                rotL(ref t);
            }
            else if (balance == 2)
            {
                if (t.lst.lst.Height - t.lst.rst.Height < 0) rotL(ref t.lst);
                rotR(ref t);
            }
            else t.Update();
        }

        T find(Node t, int k)
        {
            if (k < 0 || k > root.Count) throw new ArgumentOutOfRangeException();
            for (; ; )
            {
                if (k == t.lst.Count) return t.Key;
                else if (k < t.lst.Count) t = t.lst;
                else { k -= t.lst.Count + 1; t = t.rst; }
            }
        }
        public int LowerBound(T v)
        {
            var k = 0;
            var t = root;
            for (; ; )
            {
                if (t.Count == 0) return k;
                if (comparer.Compare(v, t.Key) <= 0) t = t.lst;
                else { k += t.lst.Count + 1; t = t.rst; }
            }
        }
        public int UpperBound(T v)
        {
            var k = 0;
            var t = root;
            for (; ; )
            {
                if (t.Count == 0) return k;
                if (comparer.Compare(t.Key, v) <= 0) { k += t.lst.Count + 1; t = t.rst; }
                else t = t.lst;
            }
        }

        void rotR(ref Node t)
        {
            var l = t.lst;
            t.lst = l.rst;
            l.rst = t;
            t.Update();
            l.Update();
            t = l;
        }
        void rotL(ref Node t)
        {
            var r = t.rst;
            t.rst = r.lst;
            r.lst = t;
            t.Update();
            r.Update();
            t = r;
        }


        class Node
        {
            public Node(T key)
            {
                Key = key;
            }
            public int Count { get; private set; }
            public sbyte Height { get; private set; }
            public T Key { get; set; }
            public Node lst, rst;
            public void Update()
            {
                Count = 1 + lst.Count + rst.Count;
                Height = (sbyte)(1 + Math.Max(lst.Height, rst.Height));
            }
            public override string ToString()
            {
                return string.Format("Count = {0}, Key = {1}", Count, Key);
            }
        }
    }
    #endregion
}

namespace CompLib.Util
{
    using System;
    using System.Linq;

    class Scanner
    {
        private string[] _line;
        private int _index;
        private const char Separator = ' ';

        public Scanner()
        {
            _line = new string[0];
            _index = 0;
        }

        public string Next()
        {
            while (_index >= _line.Length)
            {
                _line = Console.ReadLine().Split(Separator);
                _index = 0;
            }

            return _line[_index++];
        }

        public int NextInt() => int.Parse(Next());
        public long NextLong() => long.Parse(Next());
        public double NextDouble() => double.Parse(Next());
        public decimal NextDecimal() => decimal.Parse(Next());
        public char NextChar() => Next()[0];
        public char[] NextCharArray() => Next().ToCharArray();

        public string[] Array()
        {
            _line = Console.ReadLine().Split(Separator);
            _index = _line.Length;
            return _line;
        }

        public int[] IntArray() => Array().Select(int.Parse).ToArray();
        public long[] LongArray() => Array().Select(long.Parse).ToArray();
        public double[] DoubleArray() => Array().Select(double.Parse).ToArray();
        public decimal[] DecimalArray() => Array().Select(decimal.Parse).ToArray();
    }
}


Measured memory_bytes: 45892