#include <iostream>
using namespace std;
int main(){
	int n, a, m, ans = 0, sum = 0, b, c = -1;
	cin >> n;
	for(int i = 1; i <= n; i++){
		cin >> m;
		a = sum;
		sum = m;
		if(m - a == 1 && ans > c){
			ans = ans + 1;
		}else{
			c = ans;
		}
	}
	cout << ans;
	return 0;
}