Multiplying two matrices with different dimensions

I am writing an application to multiply matrices. This works nicely as intended for matrices a and b that are nxn:

for(k = 0; k < n; k++) {for(i = 0; i < n; i++) {tmp = a[i][k];for(j = 0; j < n; j++) {c[i][j] = c[i][j] + tmp * b[k][j];}}}

If a was nxy and b was yxm (implying c to be nxm). How would I modify the above loop to work?

Thanks