How is work the function ? Please explain step by step . Why is the output 20 ?Thanks in advance .

def mystery(lst):

total = 0

n = len(lst)

for i in range(n):

for j in range( i+1,n):

total += sum (lst[i:j])

return total

lst=[1,2,3,4]

print(mystery(lst))

output is 20