#include<bits/stdc++.h>
using namespace std;
const int M = 1e5 + 5;
int n, ans = 0, arr[M], s = 0;
int main() {
	scanf("%d", &n);
	arr[0] = 0;
	for (int i = 1; i < n; i++) {
		scanf("%d", &arr[i]);
		if (arr[i]-arr[i-1] ==1) {
			s++;
		}else s=1;
		if (s > ans) {
			ans = s;
		}
	}
	printf("%d", ans);
}