#pragma GCC optimize("O3")
#include <cstdio>
#include <cmath>
#include <iostream>
#include <algorithm>
#include <vector>
#include <stack>
#include <string>
#include <map>
#include <queue>
#include <unordered_map>
#include <numeric>
#include <cstring>
#include <assert.h>
#include <set>
#include <ctime>
#include <functional>
#define lowbit(x) (x & -x)
using ll = long long;
const int N = 3e5 + 5, INF = 0x3f3f3f3f;
const ll INFLL = 0x3f3f3f3f3f3f3f3fll;
const ll mod = 998244353, G = 3, Gi = 332748118;
const long double EPS = 1e-9;
namespace fast_IO {
    // char buf[N], *s, *t;
    // char getchar() {
    //     return (s == t) && (t = (s = buf) + fread(buf, 1, N, stdin)), s == t ? -1
    //     : *s++;
    // }
    ll read() {
        ll num = 0;
        char c;
        bool tag = false;
        while ((c = getchar()) != '-' && c != '+' && (c < '0' || c > '9') && ~c);
        if (!~c)return EOF;
        if (c == '-')tag = true;
        else if (c == '+')tag = false;
        else num = c ^ 48;
        while ((c = getchar()) >= '0' && c <= '9')
            num = (num << 1) + (num << 3) + (c ^ 48);
        if (tag)return -num;
        return num;
    }
    void speedup() {
        std::ios::sync_with_stdio(false);
        std::cin.tie(0); std::cout.tie(0);
    }
}
char s[N], t[N];
int cnt[500];
bool check(int cntB, int cntQ, int cntW, int m, int k) {
    int now = cntB + cntQ + cntW;
    int i = 1;
    while (now < k) {
        if (cntB > 0) {
            --cntB;
            if (t[i] == 'B')++cntB;
            else if (t[i] == 'W')++cntW;
            else if (t[i] == 'Q')++cntQ;
            ++i;
            if (i > m)break;
            if (t[i] == 'B')++cntB;
            else if (t[i] == 'W')++cntW;
            else if (t[i] == 'Q')++cntQ;
            ++i;
            if (i > m)break;
            now++;
        }
        else if (cntQ > 0) {
            --cntQ;
            if (t[i] == 'B')++cntB;
            else if (t[i] == 'W')++cntW;
            else if (t[i] == 'Q')++cntQ;
            ++i;
            if (i > m)break;
        }
        else return false;
    }
    if (i > m)return true;
    std::vector<std::vector<int>> dp(m + 1, std::vector<int>(k + 1, -1));
    --i;
    dp[i][cntB] = cntQ;
    for (; i < m; ++i) {
        for (int j = 0; j <= k; ++j) {
            if (dp[i][j] == -1)continue;
            if (j > 0) {
                auto& now = dp[std::min(i + 2, m)][j - 1 + (t[i + 1] == 'B')];
                now = std::max(now, dp[i][j] + (t[i + 1] == 'Q'));
                if (i >= m - 2)return true;
            }
            if (dp[i][j] > 0 && j + (t[i + 1] == 'B') <= k) {
                auto& now = dp[i + 1][j + (t[i + 1] == 'B')];
                now = std::max(now, dp[i][j] - 1 + (t[i + 1] == 'Q'));
                if (i == m - 1)return true;
            }
        }
    }
    return false;
}
void solve() {
    memset(cnt, 0, sizeof(cnt));
    int n, m; scanf("%d%d", &n, &m);
    scanf("%s", s + 1);
    scanf("%s", t + 1);
    int cntB = 0, cntW = 1, cntQ = 0;
    for (int i = 1; i <= n; ++i) {
        ++cnt[s[i]];
        if (s[i] == 'B')++cntB;
        else if (s[i] == 'W')++cntW;
        else if (s[i] == 'Q')++cntQ;
    }
    int l = n, r = n + m;
    while (l <= r) {
        int mid = (l + r) >> 1;
        if (check(cntB, cntQ, cntW, m, mid))r = mid - 1;
        else l = mid + 1;
    }
    if (l > n + m) {
        puts("IMPOSSIBLE");
    }
    else {
        printf("%d\n", l);
    }
}


int main() {
    // freopen("data.in", "r", stdin);
    // int ti = clock();
    int T; scanf("%d", &T);
    while (T--) {
        solve();
    }
    // std::cout << clock() - ti;
    return 0;
}