#ifndef LOCAL
#include <bits/stdc++.h>
#define debug(...)
#endif

using namespace std;
#define int long long
#define cinv(v) for (auto &it:v) cin>>it;
#define coutv(v) for (auto &it:v) cout<< it<<' '; cout<<'\n';

const int N = 2e3 + 5;
vector<bool> is_prime(N, true);
vector<int> prime{0};

void shelby() {
    int n;
    string s;
    cin >> n >> s;
    vector<int> cnt(3);
    for (auto &it: s) cnt[it - 'a']++;
    sort(cnt.begin(), cnt.end());
    debug(n, s, cnt);
    int ans = LLONG_MAX;
    auto calc = [&](int i, int j, int k)-> int {
        vector<int> v{i, j, k};
        sort(v.begin(), v.end());
        int ret = 0;
        for (int x = 0; x < 3; ++x) ret += abs(cnt[x] - v[x]);
        debug(i, j, k, ret);
        return ret / 2;
    };
    for (int i = 0; i < prime.size(); ++i) {
        for (int j = 0; j < prime.size(); ++j) {
            int k = n - prime[i] - prime[j];
            if (k >= 0 && is_prime[k]) ans = min(ans, calc(prime[i], prime[j], k));
        }
    }
    cout << (ans == LLONG_MAX ? -1 : ans) << '\n';
}

signed main() {
    cin.tie(0)->sync_with_stdio(0);
    is_prime[0] = is_prime[1] = false;
    for (int i = 2; i < N; ++i) {
        if (is_prime[i]) {
            prime.push_back(i);
            for (int j = i * i; j < N; j += i) is_prime[j] = false;
        }
    }
    int t = 1;
    cin >> t;
    for (int _ = 1; _ <= t; ++_) {
        // cout << "Case " << _ << ": ";
        shelby();
    }
}