f=function(n){
	x=rnorm(n)
	g=exp(-x^2)
 	f=mean(g)
	f
}
f(10000)

  [1] 0.5800912


 f <- function(x) {exp(-x^2)}
 integrate(f,-Inf,Inf)

1.772454 with absolute error < 4.3e-06