#include<bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
#define IOS std::ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
const long double PI = acos(-1);
const ll mod = 998244353;
const int maxn = 1e6 + 7;
const int P = 13331;

ll qgcd(ll a, ll b) {
    return b == 0 ? a : qgcd(b, a % b);
}

ll qpow(ll a, ll b) {
    ll ans = 1;
    while (b) {
        if (b & 1)
            ans = ans * a % mod;
        a = a * a % mod;
        b >>= 1;
    }
    return ans % mod;
}

int n;
int a[maxn];

void solve() {
	cin >> n;
	int flag = 1;
	int idx = 0;
	for (int i = 1; i <= n; i++) {
		cin >> a[i];
		if (a[i] != 0)
			flag = 0;
		if (a[i] == 0)
			idx = i;
	}
	if (flag) {
		cout << 0;
		return;
	} else if (idx == 0) {
		cout << -1;
		return;
	}
	int z = idx + 1;
	if (z > n)
		z = 1;
	ll cnt = a[z];
	ll y = 0;
	for (int i = idx; i <= n; i++) {
		if (a[i] != (cnt * y % n)) {
			cout << -1;
			return;
		}
		y++;
	}
	if (idx != 1)
	cnt++;
	cout << cnt;
}

int main() {
    IOS;
    int T = 1;
//    cin >> T;
    while (T--) {
        solve();
    }
    return 0;
}

/*

*/