#include 
#define N_max 123456

int n, x, y, cnt;
int a[N_max];

void init() {
	scanf("%d", &n);
	for (int i = 1; i <= n; i++) scanf("%d", &a[i]);
}

void solve() {
	for (int i = 1; i <= n; i++)
	if (a[i] > a[i+1]) {
	x = i;
	break;
	}

	if (x == n)
	y = n;
	else
	for (int i = x+1; i <= n; i++)
	if (a[i] > a[i+1]) {
	y = i;
	break;
	}

	if (x == n)
	printf("0\n");
	else if (y == n && a[y] <= a[1])
	printf("%d\n", y-x);
	else
	printf("-1\n");
}

int main() {
	init();
	solve();
}