import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		for (int i = 0; i < n; i++) {
			a[i] = sc.nextInt();
		}
		sc.close();

		long[][][] dp = new long[2000][2001][4];
		for (int i = 0; i < n; i++) {
			for (int k = 1; k < 2000; k++) {
				dp[k][a[i]][0]++;
				if (a[i] - k - 10 > 0) {
					dp[k][a[i]][1] += dp[k][a[i] - k - 10][0];
				}
				if (a[i] + k <= 2000) {
					dp[k][a[i]][2] += dp[k][a[i] + k][1];
				}
				if (a[i] - k - 1 > 0) {
					dp[k][a[i]][3] += dp[k][a[i] - k - 1][2];
				}
			}
		}
		long ans = 0;
		for (int i = 0; i < dp.length; i++) {
			for (int j = 0; j < dp[i].length; j++) {
				ans += dp[i][j][3];
			}
		}
		System.out.println(ans);
	}
}