from collections import Counter

N = int(input())
S = input()

one = 0
nine = 0
ans = 0

# 11, 19, 991

for s in S:
    if s == '3' or s == '5' or s == '7':
        ans += 1
    else:
        if s == '9':
            if one > 0:
                ans += 1
                one -= 1
            else:
                nine += 1
        elif s == '1':
            if nine >= 2:
                ans += 1
                nine -= 2
            else:
                one += 1

print(ans)