#include <cstdio>
#include <string>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <list>
#include <iterator>
#include <cassert>
#include <numeric>
#include <functional>
//#include <numeric>
#pragma warning(disable:4996) 
 
typedef long long ll;
typedef unsigned long long ull;
#define MIN(a, b) ((a)>(b)? (b): (a))
#define MAX(a, b) ((a)<(b)? (b): (a))
#define LINF  9223300000000000000
#define LINF2 1223300000000000000
#define INF 2140000000
//const long long MOD = 1000000007;
const long long MOD = 998244353;

using namespace std;


void solve()
{
    int n;
    scanf("%d", &n);
    vector<int> a;
    int prev = -1;
    int i;
    for (i = 0; i < n; i++) {
        int tmp;
        scanf("%d", &tmp);
        if (i == 0 || prev != tmp) {
            a.push_back(tmp);
        }
        prev = tmp;
    }
    int siz = (int)a.size();
    if (siz <= 2) {
        printf("0\n"); return;
    }
    int flag = 0;
    if (a[0] == a[siz - 1]) {
        flag = 1;
        siz--;
    }

    set<int> s;
    for (i = 0; i < siz; i++) {
        if (!s.insert(a[i]).second) {
            printf("-1\n"); return;
        }
    }
    printf("%d\n", flag);


    return;
}


int main(int argc, char* argv[])
{
#if 1
    solve();
#else
    int T;
    scanf("%d", &T);
    int t;
    for(t=0; t<T; t++) {
        //printf("Case #%d: ", t+1);
        solve();
    }
#endif
    return 0;
}