#include <iostream>
using namespace std;
int dp[200001][2];
int main(){
  int n;
  string s;cin>>n>>s;
  for(int i = n-1; 0 <= i; i--){
    if(s[i] == '3' || s[i] == '5' || s[i] == '7'){
      dp[i][0] = dp[i+1][0]+1;
    }else{
      if(s[i]=='9'){
        dp[i][1] = dp[i+1][1]+1;
        dp[i][0] = dp[i+1][0];
      }else{
        if(dp[i+1][1]){
          dp[i][1] = dp[i+1][1]-1;
          dp[i][0] = dp[i+1][0]+1;
        }else{
          dp[i][1] = dp[i+1][1]+1;
          dp[i][0] = dp[i+1][0];
        }
      }
    }
  }
  cout << dp[0][0] << endl;
}