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

void solve(int test)
{
    int n, m, k;
    cin >> n >> m >> k;
    vector<int> c(n);
    for(int i = 0; i < n; ++i) cin >> c[i];
    for(int i = 0; i < n; ++i)
        if(c[i] != c[(i + n - 1) % n])
        {
            rotate(c.begin(), c.begin() + i, c.end());
            break;
        }
    int prev = 0;
    vector<int> v;
    for(int i = 1; i <= n; ++i)
        if(i == n || c[i] != c[prev])
        {
            v.push_back(i - prev);
            prev = i;
        }

    if(*max_element(v.begin(), v.end()) < k) 
        cout << -1 << endl;
    else
    {
        int res = 0;
        for(int a : v) 
            res += (a + k - 1)/k;
        cout << res << endl;
    }
}

int main()
{
    int t;
    cin >> t;
    for(int i = 0; i < t; ++i)
        solve(i);
    return 0;
}