// what is matter? never mind. 
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,sse4,popcnt,abm,mmx,avx,avx2") 
#include<bits/stdc++.h>
#define For(i,a,b) for(int i=(a);i<=(b);++i)
#define Rep(i,a,b) for(int i=(a);i>=(b);--i)
#define ll long long
#define ull unsigned long long
using namespace std;
inline int read()
{
    char c=getchar();int x=0;bool f=0;
    for(;!isdigit(c);c=getchar())f^=!(c^45);
    for(;isdigit(c);c=getchar())x=(x<<1)+(x<<3)+(c^48);
    if(f)x=-x;return x;
}

#define fi first
#define se second
#define pb push_back
#define mkp make_pair
typedef pair<int,int>pii;
typedef vector<int>vi;
 
#define maxn 5005
#define inf 0x3f3f3f3f

int n,m,up;
char t[maxn],s[maxn];
int a[maxn];
int f[maxn][maxn];

void work()
{
	n=read(),m=read();
	cin>>(t+1);
	cin>>(s+1);
	For(i,1,m){
		if(s[i]=='Q')a[i]=1;
		if(s[i]=='B')a[i]=2;
		if(s[i]=='W')a[i]=0;
	}
	up=n+m;
	For(i,0,up+1) f[m+1][i]=f[m+2][i]=0;
	Rep(i,m,1){
		For(j,0,up){
			f[i][j]=inf;
			// draw 1
			if(j) f[i][j]=min(f[i][j],f[i+1][j-1+(a[i]==1)]-(a[i]==2));
			// draw 2
			if(i+2>=m) f[i][j]=min(f[i][j],1);
			else f[i][j]=min(f[i][j],f[i+2][j+(a[i]==1)]-(a[i]==2)+1);
			f[i][j]=max(f[i][j],0);
//			cout<<"i,j "<<i<<" "<<j<<" "<<f[i][j]<<"\n";
		}
	}
	int c1=0,c2=0,res=n;
	For(i,1,n){
		if(t[i]=='Q')++c1;
		if(t[i]=='B')++c2;
	}
	int pos=1;
//	cout<<"f "<<f[1][c1]<<" "<<c2<<"\n";
	if(f[pos][c1]<=c2){
		cout<<res<<"\n";
		return;
	}
	auto add=[&](){
		c1+=(a[pos]==1);
		c2+=(a[pos]==2);
		++pos;
		if(pos>m){
			cout<<res<<"\n";
			return 1;
		}
		return 0;
	};
	while(1){
		++res;
		while(c1 && pos<=m){
			--c1;
			if(add())return;
		}
		if(!c2){
			puts("IMPOSSIBLE");
			break;
		}
		--c2;
		if(add())return;
		if(add())return;
		if(f[pos][c1]<=c2){
			cout<<res<<"\n";
			return;
		}
	}
}

signed main()
{
//	freopen("data.in","r",stdin);
//	freopen("my.out","w",stdout);
	int T=read();
	while(T--)work();
    return 0;
}
/*

*/