public Matrix plus(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;
    }