package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {
	in := bufio.NewReader(os.Stdin)
	out := bufio.NewWriter(os.Stdout)
	defer out.Flush()

	var n int
	if _, err := fmt.Fscan(in, &n); err != nil {
		return
	}

	b := make([]int, n+2)
	b[0] = 0
	for i := 1; i <= n; i++ {
		fmt.Fscan(in, &b[i])
	}
	b[n+1] = 1001

	ans := 0
	for i := 0; i <= n; i++ {
		for j := i + 2; j <= n+1; j++ {
			if b[j]-b[i] == j-i {
				if j-i-1 > ans {
					ans = j - i - 1
				}
			}
		}
	}

	fmt.Fprintln(out, ans)
}