#include <stdio.h>
#include <stdlib.h>


int main(){
	int max = 2;
	int longest = 0;
	int current = 0;
	int currentSize = 0;
	int N;
	int *n;
	int i;
	while(scanf("%d", &N) > 0){
		n = (int*)malloc(N*sizeof(int));
		for(i = 0; i < N; i++){
			scanf("%d", n + i);
		}
		
		for(i = 0; i < N-2; i++){
			current = i;
			currentSize = 2;
			while(( i+2 < N ) && (n[i+1] - n[i] == n[i+2] - n[i+1])){
				currentSize++;
				i++;
			}
			if(i + 2 == N){
				if(N < 3){
					printf("%d\n", N);
				}else{
					printf("%d\n", currentSize + 1);
				}
				break;
			}
			if(currentSize > max){
				max = currentSize;
				longest = current;
			}
		}
		if((N > 2) && (n[N-3] - n[N - 2] != n[N - 2] - n[N - 1])){
			printf("%d\n", N - 1);
		}
		
	}
	return 0;
}