#include <stdio.h>

#define FOR(i, a, b) for(int (i) = (a); (i) < (b); ++(i))
#define REP(i, n) FOR(i, 0, n)

constexpr int MAX_SIZE = 2e5 + 10;
char str[MAX_SIZE];


int main(void){
    int n; scanf("%d%s", &n, str);
    int res = 0, one = 0, nine = 0;
    REP(i, n){
        switch(str[i]){
            case '1':
                one++;
                break;
            case '3':
            case '5':
            case '7':
                res++;
                break;
            case '9':
                if(one >= 1){one--; res++;}
                else nine++;
                break;
        }
    }
    int d = (one > nine / 2) ? nine / 2 : one;
    res += d; one -= d;
    res += one / 2;
    printf("%d\n", res);

    return 0;
}