#include <string>
#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <vector>

using namespace std;


int main(int argc, char ** argv)
{
	int count;
	while (cin >> count)
	{
		if( count == 0 ) return 0;
		int * stones = new int[count];
		int * visited = new int[count];
		for (int i = 0; i < count; i++) 
		{
			cin >> stones[i];
			visited[i] = 0;
		}
		//input nacten
		int vysledek = 0;
		vector<int> fronta;
		fronta.push_back(0);
		visited[0]=1;
		while(!fronta.empty()){
			int tmp = fronta.back();
			fronta.pop_back();
			for(int i=0; i<count; i++)
			{
				
				if(stones[i]+stones[tmp]==abs(i-tmp) && visited[i]==0)
				{
					fronta.push_back(i);
					visited[i]=1;
				}
			}
		}
		for(int i=0; i<count;i++)
		{
			if(visited[i]) vysledek=i;
		}
		cout << vysledek << endl;
	}	
	
	return 0;
}