n = int(input())
s = input().strip()

# Count single-digit primes (3,5,7)
count3 = s.count('3')
count5 = s.count('5')
count7 = s.count('7')
base = count3 + count5 + count7

# Build the remaining string after removing 3,5,7
remaining = []
for c in s:
    if c not in {'3', '5', '7'}:
        remaining.append(c)
s_prime = ''.join(remaining)

# Calculate additional operations for two-digit primes
additional2 = 0
count9 = 0
# Traverse from right to left
for c in reversed(s_prime):
    if c == '9':
        count9 += 1
    elif c == '1':
        if count9 > 0:
            additional2 += 1
            count9 -= 1

count1 = s_prime.count('1')
count1_remaining = count1 - additional2
additional1 = count1_remaining // 2

total = base + additional2 + additional1

print(total)