gene0 = input()
gene1 = input()
ltn = {
    'A': 0,
    'C': 1,
    'G': 2,
    'T': 3,
}
changes = [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
swaps = 0
for a, b in zip(gene0, gene1):
    if a != b:
        changes[ltn[a]][ltn[b]] += 1
for i in range(4):
    for j in range(4):
        if i != j:
            ch0 = changes[i][j]
            ch1 = changes[j][i]
            m = min(ch0, ch1)
            swaps += m
            changes[i][j] -= m
            changes[j][i] -= m
for i in range(4):
    for j in range(4):
        for k in range(4):
            if i != j and j != k and k != i:
                ch0 = changes[i][j]
                ch1 = changes[j][k]
                ch2 = changes[k][i]
                m = min(ch0, ch1, ch2)
                swaps += m * 2
                changes[i][j] -= m
                changes[j][k] -= m
                changes[k][i] -= m
swaps += max(map(max, changes)) * 3
print(swaps)