#include <bits/stdc++.h>
#pragma GCC optimize(2)
using namespace std;

typedef long long LL;
//#define int LL
#define inl inline
const int N = 3e5 + 10;
const int M = N * 2;
//const int mod=998244353;
const int mod = 1000000007;
const double eps = 1e-8;
//const int mod=1e9+7;
typedef pair<int, int> PII;
//const int INF=0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
int n, m;

void become_faster() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
}

int a[4]={-1,0,1,0};
int b[4]={0,1,0,-1};
void solve() {
    cin>>n>>m;
    if(m==1||m==2*n*(n-1)-1) {
        cout<<"Impossible"<<"\n";
        return;
    }
    if(n<=3) {
        cout<<"Possible"<<"\n";
        if(m==0) {
            for(int i=1;i<=n;i++) {
                for(int j=1;j<=n;j++)cout<<"B"<<"\n";
                cout<<"\n";
            }
            return;
        }
        if(n==2) {
            if(m==2) {
                cout<<"RB"<<"\n";
                cout<<"BB"<<"\n";
            }
            if(m==4) {
                cout<<"RB"<<"\n";
                cout<<"BR"<<"\n";
            }
            return;
        }
        if(n==3) {
            if(m==2) {
                cout<<"RBB"<<"\n";
                cout<<"BBB"<<"\n";
                cout<<"BBB"<<"\n";
            }
            if(m==3) {
                cout<<"BBB"<<"\n";
                cout<<"RBB"<<"\n";
                cout<<"BBB"<<"\n";
            }
            if(m==4) {
                cout<<"BBB"<<"\n";
                cout<<"BRB"<<"\n";
                cout<<"BBB"<<"\n";
            }
            if(m==5) {
                cout<<"RBB"<<"\n";
                cout<<"BBR"<<"\n";
                cout<<"BBB"<<"\n";
            }
            if(m==6) {
                cout<<"RBB"<<"\n";
                cout<<"BRB"<<"\n";
                cout<<"BBB"<<"\n";
            }
            if(m==7) {
                cout<<"BRB"<<"\n";
                cout<<"BBB"<<"\n";
                cout<<"RBR"<<"\n";
            }
            if(m==8) {
                cout<<"BRB"<<"\n";
                cout<<"BBR"<<"\n";
                cout<<"RBB"<<"\n";
            }
            if(m==9) {
                cout<<"BRB"<<"\n";
                cout<<"RBR"<<"\n";
                cout<<"BBB"<<"\n";
            }
            if(m==10) {
                cout<<"RRB"<<"\n";
                cout<<"RBR"<<"\n";
                cout<<"BRB"<<"\n";
            }
            if(m==12) {
                cout<<"BRB"<<"\n";
                cout<<"RBR"<<"\n";
                cout<<"BRB"<<"\n";
            }
            return;
        }
        return;
    }
    string s[n+1];
    for(int i=1;i<=n;i++) {
        for(int j=0;j<=n;j++) {
            s[i]=" "+s[i];
        }
        for(int j=1;j<=n;j++) {
            s[i][j]='B';
        }
    }
    int res=n*(n-1)*2;
    int sign=1;
    for(int i=1;i<=n;i++) {
        sign=i&1;
        sign^=1;
        for(int j=1;j<=n;j++) {
            if(sign)s[i][j]='R';
            sign^=1;
        }
    }
    vector<PII>num_2,num_3,num_4;
    for(int i=1;i<=n;i++) {
        for(int j=1;j<=n;j++) {
            if(s[i][j]=='B') {
                int cnt=0;
                for(int k=0;k<4;k++) {
                    int x=i+a[k];
                    int y=j+b[k];
                    if(x>=1&&x<=n&&y>=1&&y<=n) {
                        if(s[x][y]=='R')cnt++;
                    }
                }
                if(cnt==2)num_2.push_back({i,j});
                if(cnt==3)num_3.push_back({i,j});
                if(cnt==4)num_4.push_back({i,j});
            }
        }
    }
    /*for(int i=1;i<=n;i++) {
        for(int j=1;j<=n;j++)cerr<<s[i][j];
        cerr<<"\n";
    }*/
    //cerr<<"3: ";
    //for(auto [t1,t2]:num_3)cerr<<t1<<" "<<t2<<"\n";
    int tmp=res-m;
    //cerr<<"? tmp="<<tmp<<"\n";
    if(tmp&1) {
        if(num_3.size()) {
            tmp-=3;
            auto [x,y]=num_3.back();
            num_3.pop_back();
            s[x][y]='R';
        }
    }
    //cerr<<"tmp="<<tmp<<"\n";
    int maxn=tmp/3;
    if(maxn&1)maxn--;
    if(num_3.size()&1)num_3.pop_back();
    //cerr<<"maxn="<<maxn<<"\n";
    for(int i=0;i<min((int)num_3.size(),maxn);i++) {
        auto [x,y]=num_3[i];
        s[x][y]='R';
        tmp-=3;
    }
    //cerr<<"sb: "<<tmp<<"\n";
    maxn=tmp/4;
    for(int i=0;i<min((int)num_4.size(),maxn);i++) {
        auto [x,y]=num_4[i];
        s[x][y]='R';
        tmp-=4;
    }

    maxn=tmp/2;
    for(int i=0;i<min((int)num_2.size(),maxn);i++) {
        auto [x,y]=num_2[i];
        s[x][y]='R';
        tmp-=2;
    }
    //cerr<<"tmp="<<tmp<<"\n";
    if(tmp) {
        cout<<"Impossible"<<"\n";
        return;
    }
    cout<<"Possible"<<"\n";
    for(int i=1;i<=n;i++) {
        for(int j=1;j<=n;j++)cout<<s[i][j];
        cout<<"\n";
    }

}
/*
1
4 7

 */
signed main() {
    become_faster();
    int T = 1;
    //T=read();
    cin>>T;
    //for(int i=1;i<=100;i++)solve(i);
    while (T--) solve();
    return 0;
}