"""
3 5 7 11 19 991
"""
n = int(input())
S = input()
T = []
ans = 0
for s in S:
    if s in "357":
        ans += 1
    else:
        T.append(int(s))

S = T
T = []
for s in S:
    if s == 1:
        T.append(1)
    else:
        if T and T[-1] == 1:
            ans += 1
            T.pop()
        else:
            T.append(9)

nine = T.count(9)
one = T.count(1)

mi = min(nine // 2, one)
ans += mi
one -= mi
ans += one // 2
print(ans)