#include <bits/stdc++.h>

using namespace std;

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

int dp[200000+1];

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  int n;
  string s;
  cin >> n >> s;

  dp[n] = 0;
  for (int i = n-1; i >= 0; i--) {
    int x = s[i] - '0';
    dp[i] = dp[i+1];
    if (x == 3 || x == 5 || x == 7)
      dp[i] = dp[i+1] + 1;
    else if (x == 1) {
      if (i+2 <= n)
        dp[i] = max(dp[i], dp[i+2] + 1);
    } else {
      dp[i] = dp[i+1];
    }
  }
  cout << dp[0] << endl;
  
  return 0;
}