If f '' (x) exists, find the second derivative of function y = f (x + e ^ - x)

If f '' (x) exists, find the second derivative of function y = f (x + e ^ - x)

y=f(x+e^(-x))
y' = (1-e^(-x))f'(x+e^(-x))
y'' = e^(-x)f'(x+e^(-x)) +(1-e^(-x))^2.f''(x+e^(-x))