u = runif(2);  y = sum(-log(u)/.2)

n = 2;  m = 10^4;  lam = .2
y = replicate(m, sum(-log(runif(2))/lam))
hdr = "Simulated Dist'n of GAMMA(2, .2) with PDF"
hist(y, br=40, prob=T, col="skyblue2", main=hdr)
  curve(lam^2*x*exp(-lam*x), 0, 80, add=T, lwd=2, col="blue")
  abline(h=0, col="green3"); abline(v=0, col="green3")