a=[i for i in input()]
b=[i for i in input()]
idx={}
def removeif(d,n):
    while len(d)>0 and d[0]<n:
        del(d[0])
for i in range(len(a)):
    idx[a[i]]=[]
for i in range(len(a)):
    idx[b[i]].append(i)
sw=0
for i in range(len(a)):
    if a[i]!=b[i]:
        d=idx[a[i]]
        removeif(d,i)
        b[d[0]]=b[i]
        idx[b[i]].append(d[0])
        idx[b[i]].sort()
        b[i]=a[i]
        sw+=1
print(sw)