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=-1,Y
while l<r-1:
	m=(l+r)>>1
	if f(m)<Y:l=m
	else:r=m
print([r,-1][f(r)>Y])