Expected number of runs: $1+2n_1n_2/(n_1+n_2)$ with 20 men and 13 women

The following is an arrangement of men (M) and women (W), lined up to purchase tickets for a rock concert:

$$`MWMWMMMWMWMMMWWMMMMWWMWMMMWMMMWWW`$$

What is the expected value of runs for testing the hypothesis that the arrangement is random?