import java.util.*;

public class Main {

    public static int solution(int[] arr){
        
        int j = 0 , count = 0 , ans = 0;
        
        //acquiring loop
        for(int i = 0 ; i < arr.length ; i++){
            
            if(arr[i] == 0)
            count++;
            
            //releasing loop till count becomes valid again
                while(count > 1){
                    
                    if(arr[j] == 0)
                    count--;
                    
                    j++;
                }
            
            //update answer
            
            int len = i - j + 1;
            
            ans = Math.max(len , ans);
            
        }

        return ans;
    }
    
	public static void main(String[] args) {
		Scanner scn = new Scanner(System.in);
        int n = scn.nextInt();
        int[] arr = new int[n];
        for(int i = 0 ; i  < n; i++){
            arr[i] = scn.nextInt();
        }
        System.out.println(solution(arr));
	}

}