C: Correctly freeing memory of a multi-dimensional array

Say you have the following ANSI C code that initializes a multi-dimensional array :

int main()
{
      int i, m = 5, n = 20;
      int **a = malloc(m * sizeof(int *));

      //Initialize the arrays
      for (i = 0; i < m; i++) { 
          a[i]=malloc(n * sizeof(int));
      }

      //...do something with arrays

      //How do I free the **a ?

      return 0;
}

After using the **a, how do I correctly free it from memory ?

[Update] (Solution)

Thanks to Tim's (and the others) answer, I can now do such a function to free up memory from my multi-dimensional array :

void freeArray(int **a, int m) {
    int i;
    for (i = 0; i < m; ++i) {
        free(a[i]);
    }
    free(a);
}