using System;
using System.Collections.Generic;
using System.Linq;

namespace contest_Csharp
{
    class Program
    {
        static long power(long a, long b, long modulo)
        {
            long result = 1;
            long buff = a;
            for (long i = 1; i <= b; i *= 2)
            {
                result = result * ((b / i % (2) == 1) ? buff : 1) % modulo;
                buff = (buff * buff) % modulo;
            }
            return result % modulo;
        }
        static void Main(string[] args)
        {
            var vals = Console.ReadLine().Split(' ')
               .Select(x => int.Parse(x)).ToList();
            var N = vals[0];
            var A = Console.ReadLine().Split(' ')
                            .Select(x => int.Parse(x)).ToList();
            int large = 200000;
            long count = 0;
            long[] valset = new long[large*2];
            for (int i = 0; i < N; i++)
            {
                if (i - A[i] > -large)
                {
                    valset[i - A[i] + large]++;
                }
            }
            for (int i = 0; i < N; i++)
            {
                if(i - A[i] > -large)
                {
                    valset[i - A[i] + large]--;
                }
                if (i + A[i] < large)
                {
                    if (true){
                        count+=valset[i + A[i]+large];
                    }
                }
                

            }
            Console.WriteLine(count);


        }



    }
    class PriorityQueue<T> where T : IComparable<T>
    {
        private List<T> heap = new List<T>();
        private int lastindex = -1;
        private void swap(int n, int m)
        {
            T wait = heap[n];
            heap[n] = heap[m];
            heap[m] = wait;
        }
        public int Count()
        {
            return lastindex + 1;
        }
        public void Enqueue(T data)
        {
            int p;
            if (lastindex + 1 < heap.Count())
            {
                heap[lastindex + 1] = data;
            }
            else
            {
                heap.Add(data);
            }
            lastindex++;
            p = lastindex;

            while (true)
            {
                if (p > 0 && heap[p].CompareTo(heap[(p - 1) / 2]) <= -1)
                {
                    swap(p, (p - 1) / 2);
                    p = (p - 1) / 2;
                }
                else
                {
                    break;
                }
            }
        }
        public T Peek()
        {
            return heap[0];
        }
        public T Dequeue()
        {
            T result = Peek();
            heap[0] = default(T);
            swap(0, lastindex);
            lastindex--;
            int p = 0;
            while (true)
            {
                int np;
                if (2 * p + 1 > lastindex)
                {
                    break;
                }
                else
                {
                    np = ((2 * p + 1 == lastindex) || heap[2 * p + 1].CompareTo(heap[2 * p + 2]) <= -1)
                        ? (2 * p + 1) : (2 * p + 2);
                }
                if (heap[p].CompareTo(heap[np]) >= 1)
                {
                    swap(p, np);
                    p = np;
                }
                else
                {
                    break;
                }
            }

            return result;
        }





    }
}


Measured memory_bytes: 46760