n = 1000; x = runif(n, 0, 1); y = runif(n, 0, 1)
mean(y < x^3)

library(repr)
options(repr.plot.width=10, repr.plot.height=4)

xx = seq(0,1,length.out=100)
plot(x,y, col=1 + (y<x^3), pch=16) + lines(xx, xx^3, col="blue", lwd=3)