Y=int(input().split()[1])
S=input().split()
def f(x):
	A=[]
	for s in S:
		if"0">s:a=A.pop();A[-1]+=a
		elif"m">s:A+=[int(s)if"X">s else x]
		else:A[-2]=[min,max]["mi">s](A[-2:]);A.pop()
	return A[0]
l,r=0,Y+1
while l+1<r:
	m=(l+r)>>1
	if f(m)>Y:r=m
	else:l=m
print([l,-1][f(l)<Y])