"""
3 5 7 11 19 991
"""
n = int(input())
S = input()
lst = []
ans = 0
for s in S:
    if s in "357":
        ans += 1
    elif s == "1":
        if len(lst) >= 2 and lst[-1] == 9 and lst[-2] == 9:
            ans += 1
            lst.pop()
            lst.pop()
        else:
            lst.append(1)
    else:
        if lst and lst[-1] == 1:
            ans += 1
            lst.pop()
        else:
            lst.append(9)

c = lst.count(1)
print(ans + c // 2)