#include<bits/stdc++.h>
#define ll long long
#define vi vector<int>
#define vll vector<ll>
#define all(v) v.begin(),v.end()
#define sz(v) ((int)v.size())
#define eb emplace_back
#define pb push_back
#define pii pair<int,int>
#define pll pair<ll,ll>
#define fi first
#define se second
#define umap unordered_map
#define uset unordered_set
#define mset multiset
#define cerr if (test) cerr
#define freopen if (test) freopen
#define ui unsigned int
#define ull unsigned ll
#define i128 __int128
using namespace std;

const int test = 0;

namespace qiqi {
	const int inf = 1e9;
	const map<char,int> mp{{'Q',0},{'B',1},{'G',-1},{'W',-1}};
	void main() {
		int n,m; string s,t;
		cin >> n >> m >> s >> t;
		vector f(m+1,vi(n+m+1,inf));
		fill(all(f[m]),0);
		for (int i = m-1; i >= 0; i--) {
			array<int,2> c{};
			{
				int ch = mp.at(t[i]);
				if (ch >= 0) {
					c[ch]++;
				}
			}
			for (int j = 0; j <= n+i; j++) {
				f[i][j] = min(f[i][j],max(1,f[i+1][j+c[1]]+1-c[0]));
			}
			for (int j = 1; j <= n+i; j++) {
				f[i][j] = min(f[i][j],max(0,f[min(m,i+2)][j-1+c[1]]-c[0]));
			}
		}
		vector g(m+1,vector(n+m+1,array<int,2>{-1,-1}));
		g[0][n].fill(0);
		for (int i = 0; i < n; i++) {
			int x = mp.at(s[i]);
			if (x >= 0) {
				g[0][n][x]++;
			}
		}
		for (int i = 0; i < m; i++) {
			for (int j = n; j <= n+i; j++) {
				auto [x,y] = g[i][j];
				array<int,2> c{};
				{
					int ch = mp.at(t[i]);
					if (ch >= 0) {
						c[ch]++;
					}
				}
				if (x > 0) {
					g[i+1][j] = array<int,2>{x-1+c[0],y+c[1]};
				}
				if (i+1 < m) {
					int ch = mp.at(t[i+1]);
					if (ch >= 0) {
						c[ch]++;
					}
				}
				if (y > 0) {
					g[min(m,i+2)][j+1] = array<int,2>{x+c[0],y-1+c[1]};
				}
			}
		}
		for (int j = n; j <= n+m; j++) {
			for (int i = 0; i <= m; i++) {
				if (g[i][j] != array<int,2>{-1,-1}) {
					auto [x,y] = g[i][j];
					if (f[i][y] <= x) {
						cerr << i << ' ' << j << ' ' << x << ' ' << y << ' ' << f[i][y] << '\n';
						cout << j << '\n';
						return;
					}
				}
			}
		}
		auto fail = []() {
			cout << "IMPOSSIBLE\n";
		};
		return fail();
	}
}

int main() {
//	clock_t st = clock();
	freopen("test.in","r",stdin);
	freopen("test.out","w",stdout);
	ios::sync_with_stdio(false);
	cin.tie(nullptr);

	int T = 1;
	cin >> T;
	while (T--) {
		qiqi::main();
	}

//	cerr << (double)(clock()-st)/CLOCKS_PER_SEC;
	return 0;
}