#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;
char str[MAX_SIZE + 10];


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':
                if(nine >= 2){ nine -= 2; res++; }
                else if(one >= 1){one--; res++;}
                else one++;
                break;
            case '3':
            case '5':
            case '7':
                res++;
                break;
            case '9':
                if(one >= 1){one--; res++;}
                else nine++;
                break;
        }
    }
    
    printf("%d\n", res);
    return 0;
}