Managing failover for MySQL nodes using HA Proxy

We have an S1M2->S2 setup of MySQL replicated nodes. These are now to be brought to the back-end of an HA Proxy server to split read from writes. We also intend to achieve automatic fail-over with this. However, write requests to be routed to M2 only when M1 fails. In usual scenario, we will be good with not touching M2 at all. There seems to be no "balance" option in HA Proxy that switches to M2 only when M1 fails. Please suggest how this can be achieved using HA Proxy. Writing round-robin across M1 and M2 is a time consuming solution to be taken up at this point in time.