#include <cstdio>
#include <queue>
#include <set>
#include <cmath>

using namespace std;

int N, i, cur;
int pebbles[1000000];
queue<int> q;
set<int> s;

int main() {
	while(scanf("%d", &N) == 1 && N != 0) {
		for(i=0;i<N;i++) { scanf("%d", &pebbles[i]); }
		
		q.push(0);
		s.clear();
		s.insert(0);
		while(!q.empty()) {
			cur = q.front(); q.pop();
			for(i=0;i<N;i++) {
				if (pebbles[i] + pebbles[cur] == abs(cur-i) && s.find(i) == s.end()) {
					q.push(i);
					s.insert(i);
				}
			}
		}

		auto end = s.end();
		printf("%d\n", *(--end));
		
	}
	return 0;
}