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

cnt1 = 0
remaining = []

for c in s:
    if c in {'3', '5', '7'}:
        cnt1 += 1
    else:
        remaining.append(c)

A = [i for i, c in enumerate(remaining) if c == '1']
B = [i for i, c in enumerate(remaining) if c == '9']

i = j = y = 0
while i < len(A) and j < len(B):
    if A[i] < B[j]:
        y += 1
        i += 1
        j += 1
    else:
        j += 1

x = (len(A) - y) // 2
cnt2 = y + x

print(cnt1 + cnt2)