#include <string>
#include <iostream>

using namespace std;

int doit(int N){
	
	if(N == 1) return 1;
	
	int len = 2;
	int cur, prev, d;
	cin >> prev >> cur;
	d = cur - prev;
	prev = cur;
	
	for(int i=2; i<N; i++){
		cin >> cur;
		int cd = cur - prev;
		//cout << cur << ' ' << cd << ' ' << d << ' ' << len << endl;
		
		if(cd == d) len++;
		else len = 2;
		
		prev = cur;
		d = cd;
	}

	return N - len + 1;
}

int main(){
	int N;
	
	while(cin >> N){
		cout << doit(N) << endl;
	}
		
	return 0;
}