INPUT() doesnt appear while in expression

[Mokus]

If I use INPUT() in an expression as ([Quantity] - INPUT("Quantity to decrease", 1)) it doesn't show a dialog but silently use a default value 1. 

[Izzat_Safrah]

[Quantity] - INPUT("Quantity to decrease", 1)

App formula, not allowing user input. That why doesn't show a dialo