import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.NoSuchElementException;
import java.util.PrimitiveIterator;
import java.util.function.DoubleUnaryOperator;
import java.util.function.IntFunction;
import java.util.function.IntToLongFunction;
import java.util.function.IntUnaryOperator;
import java.util.function.LongBinaryOperator;
import java.util.function.LongPredicate;
import java.util.function.LongSupplier;
import java.util.function.LongUnaryOperator;


public class Main {
    public static void main(String[] args) {
        StringBuilder out = new StringBuilder();
        solve(out);
        PrintWriter pw = new PrintWriter(System.out);
        pw.println(out);
        pw.flush();
        pw.close();
    }

    public static void solve(StringBuilder out) {
        int n = In.ni();
        long[] a = In.nl(n);
        long[] b = new long[n];
        long[] c = new long[n];
        for (int i = 0; i < n; i++) {
            b[i] = a[i] - i;
            c[i] = -(a[i] + i);
        }
        HashMap<Long, Integer> map = new HashMap<>();
        long ans = 0;
        for (int i = 1; i < n; i++) {
            if (map.containsKey(c[i - 1])) {
                map.put(c[i - 1], map.get(c[i - 1]) + 1);
            } else {
                map.put(c[i - 1], 1);
            }
            ans += map.getOrDefault(b[i], 0);
        }
        out.append(ans);
    }
}



/**
 * @author https://atcoder.jp/users/suisen
 * 
 * (NON-DESTRUCTIVE) methods that returns long array.
 */
final class LongArrayGenerator {

    private LongArrayGenerator(){}
    
    public static long[] filled(final int n, final long init) {
        final long[] ret = new long[n];
        Arrays.fill(ret, init);
        return ret;
    }

    public static long[][] filled(final int n, final int m, final long init) {
        final long[][] ret = new long[n][m];
        for (int i = 0; i < n; i++) {
            Arrays.fill(ret[i], init);
        }
        return ret;
    }

    public static long[] generate(final int n, final LongSupplier f) {
        final long[] a = new long[n];
        Arrays.setAll(a, i -> f.getAsLong());
        return a;
    }

    public static long[][] generate(final int n, final int m, final LongSupplier f) {
        final long[][] a = new long[n][m];
        for (int i = 0; i < n; i++) {
            a[i] = generate(m, f);
        }
        return a;
    }

    public static long[] indexToLong(final int n, final IntToLongFunction f) {
        final long[] a = new long[n];
        Arrays.setAll(a, f);
        return a;
    }

    public static long[][] indexToLong(final int n, final int m, final IntToLongBiFunction f) {
        final long[][] a = new long[n][m];
        for (int i = 0; i < n; i++) {
            a[i] = indexToLong(m, f.curry(i));
        }
        return a;
    }

    public static long[] toArray(final Collection<? extends Number> collection) {
        final int n = collection.size();
        final long[] ret = new long[n];
        final Object[] coll = collection.toArray();
        for (int i = 0; i < n; i++) {
            ret[i] = ((Number) coll[i]).longValue();
        }
        return ret;
    }

    public static long[] unique(final long[] a) {
        final HashSet<Long> set = new HashSet<>();
        for (final long e : a) {
            set.add(e);
        }
        final int m = set.size();
        final long[] b = new long[m];
        int index = 0;
        for (int i = 0; i < a.length; i++) {
            if (set.contains(a[i])) {
                b[index++] = a[i];
                set.remove(a[i]);
            }
        }
        return b;
    }

    public static long[][] transpose(final long[][] a) {
        final int n = a.length;
        final int m = a[0].length;
        final long[][] ret = new long[m][n];
        for (int j = 0; j < m; j++) {
            for (int i = 0; i < n; i++) {
                ret[j][i] = a[i][j];
            }
        }
        return ret;
    }

    public static long[] map(final long[] a, final LongUnaryOperator f) {
        final long[] b = new long[a.length];
        Arrays.setAll(b, i -> f.applyAsLong(a[i]));
        return b;
    }

    public static long[] filter(final long[] a, final LongPredicate p) {
        final LongArrayList ret = new LongArrayList();
        for (final long e : a) {
            ret.addIf(e, p);
        }
        return ret.toArray();
    }

    public static long[] accumulateFromHead(final long[] a, final LongBinaryOperator op, final long e) {
        final int n = a.length;
        final long[] ret = new long[n + 1];
        ret[0] = e;
        for (int i = 0; i < n; i++) {
            ret[i + 1] = op.applyAsLong(ret[i], a[i]);
        }
        return ret;
    }

    public static long[] accumulateFromTail(final long[] a, final LongBinaryOperator op, final long e) {
        final int n = a.length;
        final long[] ret = new long[n + 1];
        ret[n] = e;
        for (int i = n - 1; i >= 0; i--) {
            ret[i] = op.applyAsLong(ret[i + 1], a[i]);
        }
        return ret;
    }
}


/**
 * @author https://atcoder.jp/users/suisen
 * 
 * 1. DESTRUCTIVE methods for long arrays.
 * 2. methods that receives arrays and return some results (except for long arrays).
 */
final class LongArrays {

    private LongArrays(){}

    public static void swap(final long[] a, final int u, final int v) {
        final long tmp = a[u];
        a[u] = a[v];
        a[v] = tmp;
    }

    public static void reverse(final long[] a, final int begin, final int end) {
        for (int i = begin; i < begin + (end - begin) / 2; i++) {
            swap(a, i, begin + end - i - 1);
        }
    }

    public static void reverse(final long[] a) {
        reverse(a, 0, a.length);
    }

    public static void sortDescending(final long[] a) {
        Arrays.sort(a);
        reverse(a);
    }

    public static long reduce(final long[] a, final LongBinaryOperator op) {
        long ret = a[0];
        for (int i = 1; i < a.length; i++) {
            ret = op.applyAsLong(ret, a[i]);
        }
        return ret;
    }

    public static void map(final long[] a, final LongUnaryOperator op) {
        Arrays.setAll(a, i -> op.applyAsLong(a[i]));
    }

    public static void filter(final long[] src, final long[] dst, final LongPredicate p) {
        int idx = 0;
        for (final long e : src) {
            if (p.test(e)) {
                dst[idx++] = e;
            }
        }
    }

    public static void filterIndex(final long[] dst, final int beginIndex, final int endIndex, final LongPredicate p) {
        for (int i = beginIndex, idx = 0; i < endIndex; i++) {
            if (p.test(i)) {
                dst[idx++] = i;
            }
        }
    }

    public static void filterIndex(final long dst[], final int endIndex, final LongPredicate p) {
        filterIndex(dst, 0, endIndex, p);
    }

    public static void accumulate(final long[] a, final LongBinaryOperator op) {
        for (int i = 1; i < a.length; i++) {
            a[i] = op.applyAsLong(a[i - 1], a[i]);
        }
    }

    public static void accumulate(final long[] a) {
        for (int i = 1; i < a.length; i++) {
            a[i] += a[i - 1];
        }
    }

    public static int unique(long[] a) {
        final HashSet<Long> set = new HashSet<>();
        for (final long e : a) {
            set.add(e);
        }
        final int m = set.size();
        final long[] b = new long[m];
        int index = 0;
        for (int i = 0; i < a.length; i++) {
            if (set.contains(a[i])) {
                b[index++] = a[i];
                set.remove(a[i]);
            }
        }
        a = b;
        return m;
    }

    public static void permute(long[] a, int[] p) {
        int n = p.length;
        boolean[] settled = new boolean[n];
        for (int i = 0; i < n; i++) {
            for (int j = i; !settled[j]; j = p[j]) {
                if (p[j] == i) {
                    settled[j] = true;
                    break;
                }
                swap(a, j, p[j]);
                settled[j] = true;
            }
        }
    }

    public static void transpose(long[][] a) {
        final int n = a.length;
        final int m = a[0].length;
        final long[][] ret = new long[m][n];
        for (int j = 0; j < m; j++) {
            for (int i = 0; i < n; i++) {
                ret[j][i] = a[i][j];
            }
        }
        a = ret;
    }

    public static int compare(final long[] a, final long[] b) {
        for (int i = 0; i < a.length; i++) {
            if (i >= b.length) {
                return -1;
            } else if (a[i] > b[i]) {
                return 1;
            } else if (a[i] < b[i]) {
                return -1;
            }
        }
        if (a.length < b.length) {
            return 1;
        } else {
            return 0;
        }
    }

    public static boolean equals(final long[] a, final long[] b) {
        return compare(a, b) == 0;
    }

    public static String join(final long[] a, final String sep) {
        final StringBuilder sb = new StringBuilder();
        for (int i = 0; i < a.length; i++) {
            sb.append(a[i]);
            if (i < a.length - 1) {
                sb.append(sep);
            }
        }
        return sb.toString();
    }

    public static String joinWithPrefixAndSuffix(final long[] a, final IntFunction<String> idxToPre,
            final IntFunction<String> idxToSuf, final String sep) {
        final StringBuilder sb = new StringBuilder();
        for (int i = 0; i < a.length; i++) {
            sb.append(idxToPre.apply(i)).append(a[i]).append(idxToSuf.apply(i));
            if (i < a.length - 1) {
                sb.append(sep);
            }
        }
        return sb.toString();
    }
}


/**
 * @author https://atcoder.jp/users/suisen
 */
final class In {
    public static final FastScanner fsc = new FastScanner();

    public static int ni() {
        return fsc.nextInt();
    }

    public static int ni(final IntUnaryOperator f) {
        return f.applyAsInt(fsc.nextInt());
    }

    public static int[] ni(final int n) {
        final int[] a = new int[n];
        Arrays.setAll(a, i -> fsc.nextInt());
        return a;
    }

    public static int[] ni(final int n, final IntUnaryOperator f) {
        final int[] a = new int[n];
        Arrays.setAll(a, i -> ni(f));
        return a;
    }

    public static int[][] ni(final int n, final int m) {
        final int[][] a = new int[n][m];
        Arrays.setAll(a, i -> ni(m));
        return a;
    }

    public static int[][] ni(final int n, final int m, final IntUnaryOperator f) {
        final int[][] a = new int[n][m];
        Arrays.setAll(a, i -> ni(m, f));
        return a;
    }

    public static long nl() {
        return fsc.nextLong();
    }

    public static long nl(final LongUnaryOperator f) {
        return f.applyAsLong(fsc.nextLong());
    }

    public static long[] nl(final int n) {
        final long[] a = new long[n];
        Arrays.setAll(a, i -> fsc.nextLong());
        return a;
    }

    public static long[] nl(final int n, final LongUnaryOperator f) {
        final long[] a = new long[n];
        Arrays.setAll(a, i -> nl(f));
        return a;
    }

    public static long[][] nl(final int n, final int m) {
        final long[][] a = new long[n][m];
        Arrays.setAll(a, i -> nl(m));
        return a;
    }

    public static long[][] nl(final int n, final int m, final LongUnaryOperator f) {
        final long[][] a = new long[n][m];
        Arrays.setAll(a, i -> nl(m, f));
        return a;
    }

    public static char[] nc() {
        return fsc.next().toCharArray();
    }

    public static char[][] nc(final int n, final int m) {
        final char[][] c = new char[n][m];
        Arrays.setAll(c, i -> nc());
        return c;
    }

    public static double nd() {
        return fsc.nextDouble();
    }

    public static double nd(final DoubleUnaryOperator f) {
        return f.applyAsDouble(fsc.nextDouble());
    }

    public static double[] nd(final int n) {
        final double[] a = new double[n];
        Arrays.setAll(a, i -> fsc.nextDouble());
        return a;
    }

    public static double[] nd(final int n, final DoubleUnaryOperator f) {
        final double[] a = new double[n];
        Arrays.setAll(a, i -> nd(f));
        return a;
    }

    public static double[][] nd(final int n, final int m) {
        final double[][] a = new double[n][m];
        Arrays.setAll(a, i -> nd(m));
        return a;
    }

    public static double[][] nd(final int n, final int m, final DoubleUnaryOperator f) {
        final double[][] a = new double[n][m];
        Arrays.setAll(a, i -> nd(m, f));
        return a;
    }

    public static String ns() {
        return fsc.next();
    }

    public static String[] ns(final int n) {
        final String[] s = new String[n];
        Arrays.setAll(s, i -> ns());
        return s;
    }

    public static boolean[][] grid(final int h, final int w, final char trueCharacter) {
        final boolean[][] grid = new boolean[h][w];
        for (int i = 0; i < h; i++) {
            final char[] s = fsc.next().toCharArray();
            for (int j = 0; j < w; j++) {
                grid[i][j] = s[j] == trueCharacter;
            }
        }
        return grid;
    }
}


final 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 (final 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(final 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();
        }
        final 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() {
        final 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());
    }
}


/**
 * @author https://atcoder.jp/users/suisen
 */
@FunctionalInterface
interface IntToLongBiFunction {
    public long apply(int x, int y);

    public default IntToLongFunction curry(final int x) {
        return y -> apply(x, y);
    }
}



/**
 * @author https://atcoder.jp/users/suisen
 */
final class LongArrayList implements Iterable<Long> {
    private long[] a;
    private int tail = 0;

    private static final int DEFAULT_SIZE = 64;

    public LongArrayList(final int capacity) {
        this.a = new long[Math.max(1, capacity)];
    }

    public LongArrayList() {
        this(DEFAULT_SIZE);
    }

    public void add(final long v) {
        if (tail == a.length) {
            resize();
        }
        a[tail++] = v;
    }

    public long removeLast() {
        return a[tail--];
    }

    public long get(final int i) {
        if (i >= a.length) {
            System.err
                    .println("Error in LongArrayList::get(" + i + "): ArrayIndexOutOfBounds. list size = " + a.length);
        }
        return a[i];
    }

    public void set(final int i, final long v) {
        if (i >= a.length) {
            System.err
                    .println("Error in LongArrayList::set(" + i + "): ArrayIndexOutOfBounds. list size = " + a.length);
        }
        a[i] = v;
    }

    private void resize() {
        final long[] b = new long[a.length << 1];
        System.arraycopy(a, 0, b, 0, a.length);
        a = b;
    }

    public int size() {
        return tail;
    }

    public void reverse(final int begin, final int end) {
        LongArrays.reverse(a, begin, end);
    }

    public void reverse() {
        LongArrays.reverse(a, 0, tail);
    }

    public long[] toArray() {
        final long[] ret = new long[tail];
        System.arraycopy(a, 0, ret, 0, tail);
        return ret;
    }

    public boolean addIf(final long v, final LongPredicate p) {
        if (p.test(v)) {
            add(v);
            return true;
        }
        return false;
    }

    public PrimitiveIterator.OfLong iterator() {
        return new LongArrayListIterator();
    }

    private class LongArrayListIterator implements PrimitiveIterator.OfLong {
        private int i = 0;
        
        public boolean hasNext() {
            return i < tail;
        }

        public long nextLong() {
            return a[i++];
        }
    }
}


Measured memory_bytes: 66216