N = int(input())
S = input()
one, prime, match = 0, 0, 0

for i in range(N):
    if S[i] in ['3', '5', '7']:
        prime += 1
    elif S[i] == '1':
        one += 1
    elif S[i] == '9':
        if one > 0:
            one -= 1
            match += 1

print(prime+match+one//2)