def lucas$($n$)$:

$"""$Returns the nth Lucas number.

Lucas numbers form a series similar to Fibonacci series,

where each number is the sum of the previous two numbers:

$2,1,3,4,7,11,18,29,47,76,123,199,322,521,843,...$

$>>>$ lucas$(0)$

$2$

$>>>$ lucas$(1)$

$1$

$>>>$ lucas$(2)$

$3$

$>>>$ lucas$(100)$

$792070839848372253127$

$"""$

$"***$ YOUR CODE HERE $***"$