electrons - Model of a particle in a linear box

I'm briefly studying the model of a particle in a linear box when we have a chain of insaturated carbons... I have a chain with 3 bonds and 4 atoms of carbon.

Applying the expression

$\lambda = \frac{8cmL^2}{ n^2 h}$ (instead of n I'm calculating the variation of n)

So make c and m the light speed and the mass of the electron, respectively and I make L the length of the box ((3+1) x distance Carbon-carbon (1.41 x 10^-10 m) and I suppose the transition with less energy is $3^2-2^2$.

I get to 210 nm and the correct answer should be 207 nm... Am I making any incorrect approach? Thanks!