VLL a,s,h,v,sh,sv;
int n,m;
void build() {
    s.assign(n+1,VL(m+1,0));
    sh=sv=h=v=s;
    for (int i=1;i<=n;i++) {
        for (int j=1;j<=m;j++) {
            h[i][j]=a[i][j]*j;
            v[i][j]=a[i][j]*i;
            s[i][j]=a[i][j]+s[i-1][j]+s[i][j-1]-s[i-1][j-1];
            sh[i][j]=h[i][j]+sh[i-1][j]+sh[i][j-1]-sh[i-1][j-1];
            sv[i][j]=v[i][j]+sv[i-1][j]+sv[i][j-1]-sv[i-1][j-1];
        }
    }
}

bool getdisk(int x,int y,int r) {
    x++;y++;
    LL hsum=sh[x+r-1][y+r-1]-sh[x-1][y+r-1]-sh[x+r-1][y-1]+sh[x-1][y-1];
    LL vsum=sv[x+r-1][y+r-1]-sv[x-1][y+r-1]-sv[x+r-1][y-1]+sv[x-1][y-1];
    LL sum=s[x+r-1][y+r-1]-s[x-1][y+r-1]-s[x+r-1][y-1]+s[x-1][y-1];
    hsum-=h[x][y]+h[x+r-1][y]+h[x][y+r-1]+h[x+r-1][y+r-1];
    vsum-=v[x][y]+v[x+r-1][y]+v[x][y+r-1]+v[x+r-1][y+r-1];
    sum-=a[x][y]+a[x+r-1][y]+a[x][y+r-1]+a[x+r-1][y+r-1];
    if (2*vsum!=(2*x+r-1)*sum) return 0;
    if (2*hsum!=(2*y+r-1)*sum) return 0;
    return 1;
}

void solve() {
    int d;
    cin >> n >> m >> d;
    a.assign(n+1,VL(m+1,0));
    for (int i=1;i<=n;i++) {
        string s;
        cin >> s;
        for (int j=1;j<=m;j++) {
            a[i][j]=s[j-1]-'0'+d;
        }
    }
    build();
    for (int i=min(n,m);i>=3;i--) {
        for (int x=0;x<=n-i;x++)
            for (int y=0;y<=m-i;y++)
                if (getdisk(x,y,i)) {
                    print(i);
                    return;
                }
    }
    print("IMPOSSIBLE");
}