#pragma omp parallel for collapse(2)
for (int i = 0; i < n; ++i)
  for (int j = 0; j < n; ++j) {
    double sum = 0;
    for (int k = 0; k < n; ++k)
      sum += A[i,k] * B[k,j];
    C[i,j] = sum;
  }