#define DEBUG 0

#include <bits/stdc++.h>
using namespace std;

#if DEBUG
// basic debugging macros
int __i__,__j__;
#define printLine(l) for(__i__=0;__i__<l;__i__++){cout<<"-";}cout<<endl
#define printLine2(l,c) for(__i__=0;__i__<l;__i__++){cout<<c;}cout<<endl
#define printVar(n) cout<<#n<<": "<<n<<endl
#define printArr(a,l) cout<<#a<<": ";for(__i__=0;__i__<l;__i__++){cout<<a[__i__]<<" ";}cout<<endl
#define print2dArr(a,r,c) cout<<#a<<":\n";for(__i__=0;__i__<r;__i__++){for(__j__=0;__j__<c;__j__++){cout<<a[__i__][__j__]<<" ";}cout<<endl;}
#define print2dArr2(a,r,c,l) cout<<#a<<":\n";for(__i__=0;__i__<r;__i__++){for(__j__=0;__j__<c;__j__++){cout<<setw(l)<<setfill(' ')<<a[__i__][__j__]<<" ";}cout<<endl;}

// advanced debugging class
// debug 1,2,'A',"test";
class _Debug {
    public:
        template<typename T>
        _Debug& operator,(T val) {
            cout << val << endl;
            return *this;
        }
};
#define debug _Debug(),
#else
#define printLine(l)
#define printLine2(l,c)
#define printVar(n)
#define printArr(a,l)
#define print2dArr(a,r,c)
#define print2dArr2(a,r,c,l)
#define debug
#endif

// define
#define MAX_VAL 999999999
#define MAX_VAL_2 999999999999999999LL
#define EPS 1e-6
#define mp make_pair
#define pb push_back

// typedef
typedef unsigned int UI;
typedef long long int LLI;
typedef unsigned long long int ULLI;
typedef unsigned short int US;
typedef pair<int,int> pii;
typedef pair<LLI,LLI> plli;
typedef vector<int> vi;
typedef vector<LLI> vlli;
typedef vector<pii> vpii;
typedef vector<plli> vplli;

// ---------- END OF TEMPLATE ----------

int a[100000],occ[100001],yes[100000],deg[100001];
int main() {
    int i;
    int t,n;
    scanf("%d",&t);
    while (t--) {
        scanf("%d",&n);
        for (i = 0; i < n; i++) scanf("%d",&a[i]);

        fill(occ,occ+n+1,0);
        for (i = 0; i < n; i++) occ[a[i]]++;
        for (i = 1; i <= n; i++) {
            if (2*occ[i] > n+1) {
                printf("-1\n");
                break;
            }
        }
        if (i > n) {
            fill(deg,deg+n+1,0);
            fill(yes,yes+n,0);
            int ans = 0;
            for (i = 1; i < n; i++) {
                if (a[i-1] == a[i]) {
                    ans++;
                    deg[a[i-1]]++;
                    deg[a[i]]++;
                }
            }
            deg[a[0]]++,yes[0] = 1;
            deg[a[n-1]]++,yes[n-1] = 1;
            int s = 0;
            for (i = 1; i <= n; i++) s += deg[i];
            int m = 0;
            for (i = 1; i <= n; i++) {
                if (2*deg[i] > s+2) m = max(m,2*deg[i]-s-2);
            }
            ans += (m+1)/2;
            printf("%d\n",ans);
        }
    }

    return 0;
}

