import java.util.*;
 
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] nums = new int[n];
 
        for (int i = 0; i < n; i++) {
            nums[i] = sc.nextInt();
        }
 
        Set<Integer> numSet = new HashSet<>();
        for (int num : nums) {
            numSet.add(num);
        }
 
        int longestSequence = 0;
 
        for (int num : nums) {
            if (!numSet.contains(num - 1)) {
                int currentNum = num;
                int currentLength = 1;
 
                while (numSet.contains(currentNum + 1)) {
                    currentNum++;
                    currentLength++;
                }
 
                longestSequence = Math.max(longestSequence, currentLength);
            }
        }
 
        System.out.println(longestSequence);
    }
}
 

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