n = int(input())
s = list(str(input()))
s.reverse()

ans = 0
cnt1 = 0
cnt9 = 0
for c in s:
    if c == '3' or c == '5' or c == '7':
        ans += 1
    elif c == '1':
        if cnt1:
            cnt1 -= 1
            ans += 1
        elif cnt9:
            cnt9 -= 1
            ans += 1
        else:
            cnt1 += 1
    else:
        if cnt1 and cnt9:
            cnt1 -= 1
            cnt9 -= 1
            ans += 1
        else:
            cnt9 += 1
print(ans)