Simple Arithmetic

How may solutions are there so that $a + b + c + d + e = 20$ where $a, b, c, d, e > -1$ ?

For example,
$a + b = 3$ have 4 solutions :
$a = 0, b = 3$
$a = 1, b = 2$
$a = 2, b = 1$
$a = 3, b = 0$