Two pairs spread moving average

Hi,
I'm looking for an indicator that plot the moving average of the spread of 2 different pairs.

For example:

GBPUUSD - EURUSD = 1.5965 - 1.4145 = 0.1820

I'd like to plot the moving average (100, 200, etc periods...) for this spread.

Any advice is well accepted.

Thank you in advance.

No error checking.
Need to enter the Pair in UpperCase.

HTH