len l[] 100
for a = 0 to 100 div 6
for b = 0 to 100 div 9
for c = 0 to 100 div 20
n = a * 6 + b * 9 + c * 20
if n >= 1 and n <= 100
l[n] = 1
.
for n = 100 downto 1
if l[n] = 0
print n
break 1