#include <tuple>
#include <iostream>
using namespace std;

void solve(){
    int n; cin >> n;
    int a[2005];
    int b[2005];
    int cc = 0, rc = 0;

    int s = 0;
    bool poss = true;
    string m; cin >> m;
    for (int i = 0; i < 2*n; i++){
        a[i] = m[i] - '0';
        s += a[i];
        if (a[0] ^ a[i] ^ (i%2))
            cc++;
    }
    if (s != n){
        poss = false;
    }

    s = a[0];
    for (int i = 1; i < 2*n; i++){
        cin >> m;
        for (int j = 0; j < 2*n; j++){
            b[j] = m[j] - '0';
        }
        s += b[0];
        if (a[0] ^ b[0] ^ (i%2))
            rc++;
        for (int j = 0; j < 2*n; j++){
            if ((a[0]^b[0]^a[j]^b[j]) != 0){
                poss = false;
            }
        }
    }
    if (!poss || s != n){
        cout << "IMPOSSIBLE\n";
        return;
    }

    cc /= 2;
    rc /= 2;

    cc = min(cc,n-cc);
    rc = min(rc,n-rc);
    cout <<cc + rc << "\n";
}

int main(void){
    ios::sync_with_stdio(0); cin.tie(0);
    int t; cin >> t;
    for (int i = 1; i <= t; i++){
        cout << "Case #" << i << ": ";
        solve();
    }
}