public Matrix minus(Matrix B) {
        Matrix A = this;
        if (B.M != A.M || B.N != A.N) throw new RuntimeException("Illegal matrix dimensions.");
        Matrix C = new Matrix(M, N);
        for (int i = 0; i < M; i++)
            for (int j = 0; j < N; j++)
                C.data[i][j] = A.data[i][j] - B.data[i][j];
        return C;
    }