def f(s,b):
m={'':'','(':')','[':']','{':'}','<':'>'}
if not s:
if len(b)>1: return "failed at: EOL"
return "yes"
i=s[0]
if i in m: b.append(i)
elif m[b[-1]]!=i: return "failed at: "+s
else: b.pop()
return f(s[1:],b)
while 1:print f(raw_input(),[''])
Note that non-ascii characters in the above source code will be escaped (such as \x9f).