#include <bits/stdc++.h>

using namespace std;

#define all(hoge) (hoge).begin(), (hoge).end()
#define en '\n'
using ll = long long;
using ull = unsigned long long;
#define rep(i, m, n) for(ll i = (ll)(m); i < (ll)(n); ++i)
#define rep2(i, m, n) for(ll i = (ll)(n)-1; i >= (ll)(m); --i)
#define REP(i, n) rep(i, 0, n)
#define REP2(i, n) rep2(i, 0, n)
template<class T> using vec = vector<T>;
template<class T> using vvec = vector<vec<T>>;
typedef pair<ll, ll> P;
using tp = tuple<ll, ll, ll>;

constexpr long long INF = 1LL << 60;
constexpr int INF_INT = 1 << 25;
constexpr long long MOD = (ll) 1e9 + 7;
//constexpr long long MOD = 998244353LL;
using ld = long double;
static const ld pi = 3.141592653589793L;

using Array = vector<ll>;
using Matrix = vector<Array>;

/*
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
*/

template<class T>
inline bool chmin(T &a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
inline bool chmax(T &a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

struct Edge {
    ll to, cap, rev;

    Edge(ll _to, ll _cap, ll _rev) {
        to = _to;
        cap = _cap;
        rev = _rev;
    }
};

typedef vector<Edge> Edges;
typedef vector<Edges> Graph;

void add_edge(Graph &G, ll from, ll to, ll cap, bool revFlag, ll revCap) {
    G[from].push_back(Edge(to, cap, (ll) G[to].size()));
    if (revFlag)
        G[to].push_back(Edge(from, revCap, (ll) G[from].size() - 1));
}

void solve() {
    int n;
    cin >> n;
    vec<int> a(n);
    REP(i, n) cin >> a[i];

    vec<int> con(n, 0);
    REP(i, n) {
        a[i]--;
        con[a[i]]++;
        if (con[a[i]] > (n + 1) / 2) {
            cout << -1 << en;
            return;
        }
    }
    int k = 0;
    vec<int> f(n, 0);
    f[a[0]]++;
    f[a[n - 1]]++;
    int ma = 0;
    REP(i, n - 1) {
        if (a[i] == a[i + 1]) {
            k++;
            f[a[i]] += 2;
            chmax(ma, f[a[i]]);
        }
    }

    cout << k + max(0, ma - (k + 2)) << en;
}

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

    ll t;
    cin >> t;
    REP(i, t - 1) {
        solve();
    }

    solve();

    return 0;
}
