#include <bits/stdc++.h>

using namespace std;

#define int long long
using i64 = long long;

typedef pair<int, int> PII;
typedef pair<int, char> PIC;
typedef pair<double, double> PDD;
typedef pair<int, PII> PIII;
typedef pair<int, pair<int, bool>> PIIB;

// const int N = 1e6 + 10;
const int maxn = 1e6 + 10;
const int mod = 998244353;
const int mod1 = 954169327;
const int mod2 = 906097321;
const int INF = 0x3f3f3f3f3f3f3f3f;
const int N = 1e5 + 10;
int a[N] , n , d[N];

void solve()
{
	cin >> n;
	for(int i = 0 ; i < n ; i ++){
		cin >> a[i];
	}
	for(int i = 0 ; i < n ; i ++){
		d[i] = (a[i] - a[(i - 1 + n) % n]);
	}
	int sum = 0 , pos = -1 , neg = 1;
	for(int i = 0 ; i < n ; i ++){
		sum += d[i];	
		if(d[i] > 0){
			if(pos == -1)pos = d[i];
			else if(pos != d[i]){
				cout << -1 << '\n';
				return;
			}
		}else if(d[i] < 0){
			if(neg == 1)neg = d[i];
			else if(neg != d[i]){
				cout << -1 << '\n';
				return;
			}
		}
	}
	
	if(sum != 0){
		cout << -1 << '\n';
		return;
	}
	int num = 0;
	for(int i = 0 ; i < n ; i ++){
		if(d[i] < 0)num ++;
	}
	if(d[n - 1] >= 0)num ++;
	cout << num << '\n';
}

signed main()
{
	ios::sync_with_stdio(false);
	cin.tie(0), cout.tie(0);

	int t = 1;
	// cin >> t;
	while (t--)
	{
		solve();
	}
}