package main3;
import java.util.Arrays;
import java.util.Scanner;
public class Main3 {
public int solution(int n, int m , int []arr1){
int count = 0;
int answer = 0 ;
int lt = 0;
for(int rt = 0 ; rt < n; rt ++) {
if((arr1[rt]) == 0 ) {
count ++;
}
while(count > 2) {
if(arr1[lt] == 0) {
count --;
lt ++;
}
answer = Math.max(answer, rt-lt + 1);
}
}
return answer;
};
public int[] split(int []arr1) {
for(int i = 0 ; i < arr1.length; i ++ ) {
if(arr1[i] == 0 ) {
int[] b = Arrays.copyOfRange(arr1, i+1, arr1.length);
return b;
}
}
return null;
}
public static void main(String[] args) {
Main3 main2 = new Main3();
Scanner kb = new Scanner(System.in);
int n=kb.nextInt();
int m=kb.nextInt();
int []arr1 = new int[n];
for(int i=0; i<n; i++){
arr1[i] = kb.nextInt();
};
main2.solution(n,m,arr1);
}
}