import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int pre = -1,mark = 1,max = 1;
        for (int i = 0; i < n; i++) {
            int x = sc.nextInt();
            if (x-pre==1){
                mark++;
            }else {
                if (mark>max){
                    max = mark;
                }
                mark = 1;
            }
            pre = x;
        }
        System.out.println(max);
    }
}