#include<bits/stdc++.h>
#include <vector>

#define range(x) begin(x), end(x)

using namespace std;

typedef long long ll;

typedef pair<int, int> pii;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int t;
    cin >> t;


    while (t--) {
        int n, k;
        cin >> n >> k;
        vector<vector<int>> block(n, vector<int>(n));
        vector<pair<int, pii>> op;
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                if ((i + j) % 2 == 0) {
                    block[i][j] = 1;
                    int cnt = 4;
                    if (i == 0 || i == n - 1) cnt--;
                    if (j == 0 || j == n - 1)cnt--;
                    op.push_back({cnt, {i, j}});
                }
            }
        }
        sort(op.begin(), op.end());
        reverse(op.begin(), op.end());
        ll num = 2LL * n * (n - 1) - k;
        for (auto [cnt, pos]: op) {
            if (num - cnt != 1 && num >= cnt) {
                num -= cnt;
                block[pos.first][pos.second] = 0;
            }
        }

        if(num != 0 ){
            cout <<"Impossible\n";
        }
        else{

            cout <<"Possible\n";

            for(int i=0;i<n;i++){
                for(int j=0;j<n;j++){
                    cout << (block[i][j]?'R':'B');
                }
                cout <<"\n";
            }
            
        }
    }
    return 0;
}