def calculate$\_$light$\_$travel$\_$distance$($years$)$:

$"""$

This program calculates the distance traveled by light in a given number of years.

Inputs:

$-$ years: Number of years light has traveled.

Output:

$-$ The distance traveled by light in those years, in meters.

Method:

$-$ The program multiplies the number of years by the speed of light $(3$ x $10^8$ meters$/$second$)$ and the number of seconds in a year.

$-$ It ensures that the input is valid $($a positive number$)$ and then performs the calculation.

$"""$

# Constants

speed$\_$of$\_$light $=3*10**8$ # meters$/$second

seconds$\_$in$\_$year $=365\mathrm{.}25*24*60*60$ # seconds$/$year

# Calculate the distance

distance $=$ years $*$ seconds$\_$in$\_$year $*$ speed$\_$of$\_$light

return distance

# Get user input

years $=$ float$($input$("$Enter the number of years: $"))$

# Validate input and calculate distance

if years $<0$:

print$("$Error: Number of years must be a positive number."$)$

else:

distance $=$ calculate$\_$light$\_$travel$\_$distance$($years$)$

print$($f$"$Light travels $\{$distance:$\mathrm{.}2$e$\}$ meters in $\{$years$\}$ years."$)$