I'm trying to determine a sum via a number of SUMIF's, and am failing badly !

To explain, I have 4 columns with data in - Names, Town, Month, Sales. I
need to show the total Sales number where the Names and Town and Month are
the same.

eg
=sumif(a:a="Smith",sumif(b:b="London",sumif(c:c="J an",(d:d))))

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Many thanks

Ian