#include <bits/stdc++.h>

int ri() {
	int n;
	scanf("%d", &n);
	return n;
}

// should be TLE

int main() {
	int n = ri();
	int a[n];
	for (auto &i : a) i = ri() - 1;
	
	int res = -1;
	for (int i = 0; i < n; i++) {
		int used[n];
		memset(used, 0, sizeof(used));
		bool ok = true;
		for (int j = 0; j < n; j++) {
			if (j && a[j] == a[j - 1]) continue;
			if (used[a[j]]) {
				ok = false;
				break;
			}
			used[a[j]] = true;
		}
		if (ok) {
			res = !!i;
			break;
		}
		std::rotate(a, a + 1, a + n);
	}
	printf("%d\n", res); 
	return 0;
}