#include<bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(),x.end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define INF 1000000000
#define mod 1000000007
using ll=long long;
const ll LINF=1001002003004005006ll;
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}

// 3,5,7
// 11,19,

signed main(){
    cin.tie(0);
    ios::sync_with_stdio(0);

    int n;cin>>n;
    string s;cin>>s;
    stack<int> st;
    int ans=0;
    for(auto c:s){
        int num=c-'0';
        if(num==3 or num==5 or num==7){
            ans++;
            continue;
        }
        if(!st.empty() and st.top()==1){
            ans++;
            st.pop();
        }else{
            st.push(num);
        }
    }
    cout<<ans<<endl;
    return 0;
}