import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.math.BigInteger;
import java.util.*;

public class Main {

    public static void main(String[] args) throws IOException {
        int n = GetInput.getInt();
        long []ins = GetInput.getLongArrayInput();

        HashMap<Long, Long> frequency = new HashMap<>();

        for (int i = 0 ; i < n ; i++){
            if (frequency.containsKey(i+ins[i])){
                frequency.replace(i+ins[i], frequency.get(i+ins[i]) + 1);
                continue;
            }
            frequency.put(i+ins[i], (long) 1);
        }

        long count = 0;
        for (int i = 0; i < n; i++){
            if (frequency.containsKey(i-ins[i])){
                count+=frequency.get(i-ins[i]);
            }
        }

        System.out.println(count);
    }

    //        int n = ins[0]; int m = ins[1]; int q = ins[2];
//        int [][]table = new int[n][n];
//
//        int [][]tuples = new int[q][];
//        for (int i = 0 ; i< q; i++){
//            tuples[i] = GetInput.getArrayInt();
//            if (table[tuples[i][0]][tuples[i][1]] < tuples[i][3]){
//                table[tuples[i][0]][tuples[i][1]] = tuples[i][3];
//            }
//        }
//        for (int i = 0 ; i < n; i++){
//            for (int j = 0; j < n; j++){
//
//            }
//        }

//    int n = GetInput.getInt();
    //        int []heigts = GetInput.getArrayInt();
//
////        int sum = 0;
////        for (int i = 0; i < n; i++){
////            for (int j = i; j < n; j++){
////                if (j-i == heigts[i] + heigts[j]){
////                    sum++;
////                }
////            }
////        }
////        System.out.println(sum);
    private static Set<Integer> isBeut(int[] arrau, int k) {
        int max = Integer.MIN_VALUE;
        int min = Integer.MAX_VALUE;
        int distinct = 0;
        Set<Integer> set = new HashSet<>();
        for (int i = 0; i < arrau.length; i++) {
            if (arrau[i] > max) {
                max = arrau[i];
            }
            if (arrau[i] < min) {
                min = arrau[i];
            }

            if (!set.contains(arrau[i])) {
                distinct++;
                set.add(arrau[i]);
            }
        }
        return set;

    }

    private static long sumUpto(int n) {
        return (long) (Math.pow(2, n + 1) - 2);
    }

    private static long mod(long position, long position1) {
        if (position - position1 > 0) {
            return position - position1;
        }
        return position1 - position;
    }

}


class GetInput {
    static BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));

    static char[] getCharArray() {
        char[] charArray;
        try {
            StringBuilder string = getInputString();
            charArray = new char[string.length()];
            for (int i = 0; i < string.length(); i++) {
                charArray[i] = string.charAt(i);
            }
            return charArray;
        } catch (Exception e) {
            e.printStackTrace();
        }
        charArray = new char[0];
        return charArray;
    }

    static int[] getArrayInt() {
        String[] string;
        int[] array;
        try {
            string = bufferedReader.readLine().split("\\s+");
            array = new int[string.length];
            for (int i = 0; i < string.length; i++) {
                array[i] = Integer.parseInt(string[i]);
            }
            return array;
        } catch (IOException e) {
            e.printStackTrace();
        }

        int[] arra = new int[2];
        return arra;
    }

    static int getInt() {
        try {
            String string = bufferedReader.readLine();
            return Integer.parseInt(string);
        } catch (IOException e) {
            e.printStackTrace();
        }

        return 0;
    }

    static StringBuilder getInputString() {
        try {
            StringBuilder string = new StringBuilder();
            string.append(bufferedReader.readLine());
            return string;
        } catch (IOException e) {
            e.printStackTrace();
        }
        return new StringBuilder();
    }

    static long getLongInput() {
        try {
            String string = bufferedReader.readLine();
            return Long.parseLong(string);
        } catch (IOException e) {
            e.printStackTrace();
        }

        return 0;
    }

    static long[] getLongArrayInput() {
        String[] string;
        long[] array;
        try {
            string = bufferedReader.readLine().split("\\s+");
            array = new long[string.length];
            for (int i = 0; i < string.length; i++) {
                array[i] = Long.parseLong(string[i]);
            }
            return array;
        } catch (IOException e) {
            e.printStackTrace();
        }

        long[] arra = new long[2];
        return arra;
    }
}


Measured memory_bytes: 73684