f(x) = exp(-0.5 * ((x - 12) / 2)^(2)) / (2 * sqrt(2 * 3.141592653589793))
a = 9.5 (slider from 0 to 25, step 0.5)
b = 13.5 (slider from 0 to 25, step 0.5)
aire = Integral(f, a, b)