It will run at the default speed that your motherboard sets. Yes many people will recommend getting matching sticks of RAM, but I almost never run into issues and I mix and match RAM all the time. For example, If you put just one 1866MHz RAM into your motherboard, it will still run at 1333MHz by default until you go into the BIOS and overclock it to 1866MHz.
The speed that is labeled on all sticks of RAM is just the highest that the stick of RAM is "rated" to run by the manufacturer, and usually has the XMP profile set to that for easy overclocking. Meaning you could run that stick at any speed up to what the stick says.