Plotting – How to represent characteristic functions of a one-point set?


I try to find out how to represent the characteristic function of a one-point set. My idea was to write the points by sentence:

Ainput = {{2/5, 1/4}};

ListPlot[Ainput, PlotRange -> {{0, 1}, {0, 1}}]

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Now I want to find a y-coordinate for points

{{x -> 2/5}, {x -> 3/5}}

But since I do not have Ainput defined as a function, both results show 0 results instead of 1 / 4.0. Do you have suggestions on how I could define the function or how I could find the y-coordinate? Many Thanks.