#include<bits/stdc++.h>
#define fi first
#define se second
#define mkp make_pair
#define pii pair<int,int>
#define ll long long
using namespace std;
const int maxn=3010,lim=1e9;
string s0,s;
bool f[maxn][maxn];
int g[maxn][maxn];
int c1[maxn],c2[maxn];
int main() {
	ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);	
	memset(g,63,sizeof(g));
	int T;cin>>T;
	while (T--) {
		int n,m,k,ans=lim;
		cin>>n>>m;
		cin>>s0>>s;s="0"+s+"WW";
		for (int i=0;i<s0.size();i++) c1[0]+=(s0[i]=='Q'),c2[0]+=(s0[i]=='B');
		for (int j=1;j<=m+1;j++) c1[j]=c1[j-1]+(s[j]=='Q'),c2[j]=c2[j-1]+(s[j]=='B');
		f[0][0]=1;
		for (int i=0;i<m;i++) {
			for (int j=0;j<=c2[i];j++) {
				if (f[i][j]==0) continue;
				if (j!=c2[i]) f[i+2][j+1]=1;
				assert(i-2*j>=0);if (c1[i]>i-2*j) f[i+1][j]=1;
			}
		}
		for (int j=0;j<=c2[m];j++) {
			g[m][j]=0;g[m+1][j]=0;
		}
		for (int i=m-1;i>=0;i--) {
			for (int j=0;j<=c2[i];j++) {
				g[i][j]=lim;
				if (j!=0) g[i][j]=min(g[i][j],
					max(0,g[i+2][j-1+c2[i+1]-c2[i]]-c1[i+1]+c1[i]));
				g[i][j]=min(g[i][j],max(0,g[i+1][j+c2[i+1]-c2[i]]-c1[i+1]+c1[i])+1);
			}
		}
		for (int i=0;i<=m+1;i++) {
			for (int j=0;j<=c2[i];j++) {
				if (f[i][j]&&g[i][c2[i]-j]<=i-2*j) {
				//	cout<<i<<" "<<j<<" : "<<c2[i]-j<<" "<<g[i][c2[i]-j]<<endl;
					ans=min(ans,n+j);
				}
			}
		}
		for (int i=0;i<=m+1;i++) {
			for (int j=0;j<=c2[i];j++) {
				f[i][j]=0;g[i][j]=lim;
			}
		}
		for (int i=0;i<=m+1;i++) c1[0]=c2[0]=0;
		if (ans>=lim) cout<<"IMPOSSIBLE\n";
		else cout<<ans<<endl;
	}
}
/*
2
4 6
1 1 8
7 2 5
1 1 7
3 2 6
8 1200000
100000 1 100000
100000 1 12345
100000 2 100000
100000 2 12345
100000 1 100000
100000 1 12345
100000 2 100000
100000 2 12345


*/