#include <bits/stdc++.h>
#define fp(i, a, b) for (int i = a, i##_ = int(b); i <= i##_; ++i)
#define fd(i, a, b) for (int i = a, i##_ = int(b); i >= i##_; --i)

using namespace std;
using ll = long long;
const int N = 5e3 + 5;
int n, m;
char s[N], t[N];
bool Chk(int k, int cntQ, int cntB) {
    int i = 1, cur = n;
    auto upd = [&]() {
        if (t[i] == 'Q') ++cntQ;
        else if (t[i] == 'B') ++cntB;
        ++i;
    };
    while (cur < k && i <= m) {
        if (cntB > 0)
            ++cur, --cntB, upd(), upd();
        else if (cntQ > 0)
            ++i, --cntQ, upd();
        else return false;
    }
    if (i > m) return true;
    vector<vector<int>> f(m + 1, vector<int>(k + 1, -1));
    f[--i][cntB] = cntQ;
    for (; i < m; ++i) fp(j, 0, k) if (~f[i][j]) {
        if (j > 0) {
            auto& now = f[min(i + 2, m)][j - 1 + (t[i + 1] == 'B')];
            now = max(now, f[i][j] + (t[i + 1] == 'Q'));
            if (i >= m - 2) return true;
        }
        if (f[i][j] > 0 && j + (t[i + 1] == 'B') <= k) {
            auto& now = f[i + 1][j + (t[i + 1] == 'B')];
            now = max(now, f[i][j] + (t[i + 1] == 'Q') - 1);
            if (i == m - 1) return true;
        }
    }
    return false;
}
void Solve() {
    scanf("%d%d", &n, &m);
    scanf("%s%s", s + 1, t + 1);
    int cntB = 0, cntQ = 0;
    fp(i, 1, n) {
        if (s[i] == 'Q') ++cntQ;
        if (s[i] == 'B') ++cntB;
    }
    int l = n, r = n + m, ans = -1;
    while (l <= r) {
        int mid = (l + r) >> 1;
        if (Chk(mid, cntQ, cntB)) ans = mid, r = mid - 1;
        else l = mid + 1;
    }
    if (ans == -1) puts("IMPOSSIBLE");
    else printf("%d\n", ans);
}
int main() {
    int t = 1;
    scanf("%d", &t);
    while (t--) Solve();
    return 0;
}