// yuki 915 Plus Or Multiple Operation
// 2019.11.16 bal4u

#include <stdio.h>
#include <ctype.h>

int getchar_unlocked(void);
#define gc() getchar_unlocked()

int in() {   // 非負整数の入力
	int n = 0; int c;
	do c = gc(); while (isspace(c));
	do n = 10 * n + (c & 0xf); while ((c = gc()) >= '0');
	return n;
}

int ans;

void ins(char *s) { // 特殊：文字列の入力
    char c;
	while ((c = gc()) > ' ') {
        if (c == '3' || c == '5' || c == '7') ans++;
        else *s++ = c;
    }
	*s = 0;
}

char S[200005];

int main()
{
	int n1, n9, t;
    char *p;
	
    in(), ins(S);
    n1 = n9 = 0, p = S;

    while (*p) {
        if (*p == '1') n1++;
        else {
            if (n1) ans++, n1--;
            else n9++;
        }
        p++;
    }
    if (n1 <= n9/2) ans += n1;
    else {
        ans += n9/2, n1 -= n9/2;
        ans += n1/2;
    }
    printf("%d\n", ans);
	return 0;
}