import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.InputMismatchException;

public class Main {

  public static void main(String[] args) {
    InputReader in = new InputReader(System.in);

    int n = in.nextInt();
    int[] h = new int[n+1];
    HashMap<Integer, ArrayList<Integer>> map = new HashMap<>();
    HashMap<Integer, ArrayList<Integer>> map1 = new HashMap<>();
    long count = 0;
    for (int i = 1; i <= n; i++) {
      h[i] = in.nextInt();
      if (!map.containsKey(h[i] + i)) {
        map.put(h[i]+i, new ArrayList<>());
      }
      if (!map1.containsKey(i - h[i])) {
        map1.put(i-h[i], new ArrayList<>());
      }
      map.get(h[i]+i).add(i);
      map1.get(i-h[i]).add(i);
    }

    for (int i = 1; i < n; i++) {
      if (map.containsKey(i) && map1.containsKey(i)) {
        count += (long)map.get(i).size() * map1.get(i).size();
      }
    }
    System.out.println(count);
  }

  static class InputReader {

    private final InputStream stream;
    private final byte[] buf = new byte[8192];
    private int curChar, snumChars;
    private SpaceCharFilter filter;

    public InputReader(InputStream stream) {
      this.stream = stream;
    }

    public int snext() {
      if (snumChars == -1) throw new InputMismatchException();
      if (curChar >= snumChars) {
        curChar = 0;
        try {
          snumChars = stream.read(buf);
        } catch (IOException e) {
          throw new InputMismatchException();
        }
        if (snumChars <= 0) return -1;
      }
      return buf[curChar++];
    }

    public int nextInt() {
      int c = snext();
      while (isSpaceChar(c)) {
        c = snext();
      }
      int sgn = 1;
      if (c == '-') {
        sgn = -1;
        c = snext();
      }
      int res = 0;
      do {
        if (c < '0' || c > '9') throw new InputMismatchException();
        res *= 10;
        res += c - '0';
        c = snext();
      } while (!isSpaceChar(c));
      return res * sgn;
    }

    public long nextLong() {
      int c = snext();
      while (isSpaceChar(c)) {
        c = snext();
      }
      int sgn = 1;
      if (c == '-') {
        sgn = -1;
        c = snext();
      }
      long res = 0;
      do {
        if (c < '0' || c > '9') throw new InputMismatchException();
        res *= 10;
        res += c - '0';
        c = snext();
      } while (!isSpaceChar(c));
      return res * sgn;
    }

    public int[] nextIntArray(int n) {
      int a[] = new int[n];
      for (int i = 0; i < n; i++) {
        a[i] = nextInt();
      }
      return a;
    }

    public String readString() {
      int c = snext();
      while (isSpaceChar(c)) {
        c = snext();
      }
      StringBuilder res = new StringBuilder();
      do {
        res.appendCodePoint(c);
        c = snext();
      } while (!isSpaceChar(c));
      return res.toString();
    }

    public String nextLine() {
      int c = snext();
      while (isSpaceChar(c)) c = snext();
      StringBuilder res = new StringBuilder();
      do {
        res.appendCodePoint(c);
        c = snext();
      } while (!isEndOfLine(c));
      return res.toString();
    }

    public boolean isSpaceChar(int c) {
      if (filter != null) return filter.isSpaceChar(c);
      return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

    private boolean isEndOfLine(int c) {
      return c == '\n' || c == '\r' || c == -1;
    }

    public interface SpaceCharFilter {
      public boolean isSpaceChar(int ch);
    }
  }
}


Measured memory_bytes: 110500