sum of different values added one by one in a cell ?

hello . I just stumbled upon an interesting problem in excel .
I hope you can understand
so . .
I want to calculate the sum of different numbers that are entered one by one is a cell , and the sum of all of those to be displayed in a different one .

so , for example :

in cell A1 I put the number 2 .
in cell C1 the sum will be ... 2 of course .
now .. in cell A1 I replace 2 with another number , lets say 5

in cell C1 the sum will be changed to 7 .

I hope this is clear

Can someone help me or tell at least tell me is this is possible ?
Thanks !

put this code in your worksheet module

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how do I do that ? sorry . I dont really use excel .
thanks !