Calculating purchase price of shares bought over time

I worked for an American company in the uk and bought employee shares in the US by monthly standing order over a few years in the 1990s. The dividends were then automatically reinvested (and still are ) to buy more partial shares. I am now retired and wish to sell them. I realise I have to to pay capital gains tax, but how is the original purchase price of the shares calculated, since the purchase was over a number of years including those purchased with dividends?

Hi,
Please refer to guidance at
Shares and Capital Gains Tax (Self Assessment helpsheet HS284)
Thank you.