View Compilation Error

0_0_21171615_3487.cpp: In member function 'Mat Mat::operator*(const Mat&) const':
0_0_21171615_3487.cpp:32:37: error: no match for 'operator[]' (operand types are 'const Mat' and 'int')
           res.a[i][j] += a[i][k] * b[k][j];
                                     ^
0_0_21171615_3487.cpp: In function 'll calc(int, Mat&)':
0_0_21171615_3487.cpp:120:13: error: no match for 'operator[]' (operand types are 'Mat' and 'int')
   ll res = B[1][0] * K;
             ^