n = int(input())
s = input()

from collections import defaultdict

count = defaultdict(int)
for c in s:
    count[c] += 1

ans = 0

# Step 1: Count 1-digit primes (3, 5, 7)
ans += count['3'] + count['5'] + count['7']
count['3'] = 0
count['5'] = 0
count['7'] = 0

# Step 2: Form as many 11 as possible
x = count['1']
ans += x // 2
remaining_ones = x % 2
count['1'] = remaining_ones

# Step 3: Form primes with 1 and 9 (19)
possible_pairs = min(count['1'], count['9'])
ans += possible_pairs

print(ans)