#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <climits>
#include <cmath>
#include <complex>
#include <cstring>
#include <deque>
#include <functional>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <vector>
#include <cstdint>
using namespace std;
typedef long long ll;
#define MP make_pair
#define PB push_back
#define inf 1000000007
#define mod 1000000007
#define rep(i,n) for(int i = 0; i < (int)(n); ++i)

int main(){
    int n;
    string s;
    cin >> n >> s;
    int a = 0;
    int b = 0;
    int sm = 0;
    rep(i,n){
        if(s[i]=='1'){
            if(b>=2){
                b-=2;
                sm++;
            }else{
                a++;
            }
        }else if(s[i]=='9'){
            if(a>=1){
                a--;
                sm++;
            }else{
                b++;
            }
        }else{
            sm++;
        }
    }
    sm += a/2;
    cout << sm << endl;
    return 0;
}