#include <bits/stdc++.h>

using namespace std;

const int N = 2510;
const int INF = 1e9;

int is1[N], is2[N], pre1[N], pre2[N];
bool dp1[N][2 * N];
int dp2[N][2 * N];

inline bool valid1(int cnt, int x) { return (cnt >= 0) && (cnt <= pre1[x]); }
inline bool valid2(int cnt, int x) { return (cnt >= 0) && (cnt <= pre2[x]); }
void init(int n, int m);
void solve(int n, int m);
void solve_dp1(int n, int m);
void solve_dp2(int n, int m);
bool check(int n, int m, int volume);

int main()
{
    ios::sync_with_stdio(false), cin.tie(0), cout.tie(0);
    int t, n, m;
    cin >> t;
    while (t--)
    {
        cin >> n >> m;
        init(n, m);
        solve(n, m);
    }
    return 0;
}

void init(int n, int m)
{
    string card;
    cin >> card;
    pre1[0] = pre2[0] = 0;
    for (auto c : card)
    {
        if (c == 'Q')
            pre1[0]++;
        else if (c == 'B')
            pre2[0]++;
    }
    cin >> card;
    for (int i = 1; i <= m; i++)
    {
        is1[i] = (card[i - 1] == 'Q');
        pre1[i] = pre1[i - 1] + is1[i];
        is2[i] = (card[i - 1] == 'B');
        pre2[i] = pre2[i - 1] + is2[i];
    }
    return;
}

void solve(int n, int m)
{
    solve_dp1(n, m);
    solve_dp2(n, m);
    for (int v = n; v <= n + m; v++)
        if (check(n, m, v))
        {
            cout << v << endl;
            return;
        }
    cout << "IMPOSSIBLE" << endl;
    return;
}

void solve_dp1(int n, int m)
{
    for (int i = 0; i <= m; i++)
        for (int j = 0; j <= pre2[i]; j++)
            dp1[i][j] = false;
    dp1[0][pre2[0]] = true;
    for (int i = 0; i < m; i++)
        for (int j = 0; j <= pre2[i]; j++)
        {
            if (!dp1[i][j])
                continue;
            if (pre1[i] - (i - 2 * (pre2[i] - j)) > 0)
                dp1[i + 1][j + is2[i + 1]] = true;
            if (j > 0)
            {
                if (i == m - 1)
                    dp1[i + 1][j - 1 + is2[i + 1]] = true;
                else
                    dp1[i + 2][j - 1 + is2[i + 1] + is2[i + 2]] = true;
            }
        }
    return;
}

void solve_dp2(int n, int m)
{
    for (int j = 0; j <= pre2[m]; j++)
        dp2[m][j] = 0;
    for (int i = m - 1; i >= 0; i--)
    {
        dp2[i][0] = max(1, dp2[i + 1][is2[i + 1]] + 1 - is1[i + 1]);
        for (int j = 1; j <= pre2[i]; j++)
        {
            dp2[i][j] = max(1, dp2[i + 1][j + is2[i + 1]] + 1 - is1[i + 1]);
            dp2[i][j] = min(dp2[i][j], dp2[min(i + 2, m)][j - 1 + is2[i + 1]] - is1[i + 1]);
        }
    }
    return;
}

bool check(int n, int m, int volume)
{
    int cnt1, cnt2;
    for (int t = 0; t <= m; t++)
    {
        cnt1 = pre1[t] - (t - 2 * (volume - n));
        cnt2 = pre2[t] - (volume - n);
        if (valid1(cnt1, t) && valid2(cnt2, t) && dp1[t][cnt2] && (dp2[t][cnt2] <= cnt1))
        {
            // cout << "volume = " << volume << ", t = " << t << endl;
            // cout << "cnt1 = " << cnt1 << ", cnt2 = " << cnt2 << endl;
            // cout << "dp1 = " << dp1[t][cnt2] << ", dp2 = " << dp2[t][cnt2] << endl;
            return true;
        }
    }
    return false;
}