#include <iostream>
#include <vector>

using namespace std;

int main()
{
	int N;
	cin >> N;
	
	vector <int> x(N);
	vector <int> y(N, 0);
	for (int i = 0; i < N; ++i) cin >> x[i];

	int r = 0, h_db=N;
	int i = 0;

	while (h_db > 0)
	{
		if (y[(x[i] + r) % N] == 0) --h_db;
		y[(x[i] + r) % N] = 1;
		while (y[i] == 1)
		{
			++i;
			if (i == N) { h_db = 0; break; }
			
			if (y[(x[i] + r) % N] == 0) --h_db;
			y[(x[i] + r) % N] = 1;
		}

		++r;
	}

	cout << r - 1;

	return 0;
}