src = [0,'A',1,'A',3,'A',5,'B',8,'A',13,'A',20,'A']
dst = []
sep = src[src.index('B')-1]
for i in range(0, len(src), 2):
    if src[i]>=sep:
        dst.append(src[i])
        dst.append(src[i+1])
print dst