MOD = 10 ** 9 + 7
INF = 10 ** 10
import sys
sys.setrecursionlimit(100000000)
dy = (-1,0,1,0)
dx = (0,1,0,-1)

def main():
    n = int(input())
    s = input()
    stack = [-1]
    ans = 0
    for i in range(n):
        if s[i] in '357':
            ans += 1
        else:
            if s[i] == '9':
                if stack[-1] == '1':
                    ans += 1
                    stack.pop()
            else:
                stack.append(s[i])
    
    ans += stack.count('1')//2
    print(ans)

if __name__ =='__main__':
    main()