Add a number to a variable when I press a button

Hello, I would like to know how to add a number to the right of the value of a variable and update this variable with the new value.

example at the start my variable is empty, when I click on 1 my variable goes to 1 when I click on 2 my variable goes to 12 then I click on 5 and my variable goes to 125

thanks in advance

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