#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) begin(v),end(v)
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
using ll = long long;
using pii = pair<int, int>;
constexpr ll INF = 1ll<<30;
constexpr ll longINF = 1ll<<60;
constexpr ll MOD = 1000000007;
constexpr bool debug = false;
//---------------------------------//

int main() {
	int N;
	cin >> N;
	string S;
	cin >> S;
	// 11, 19, 991
	int ans = 0, s = 0;
	for (char c : S) {
		const int n = c - '0';
		if (n == 3 || n == 5 || n == 7) ++ans;
		else {
			if (s == 1) { ++ans; s = 0; }
			else if (s <= -2 && n == 1) { ++ans; s = 0; }
			else if (n == 1) s = 1;
			else --s;
		}
	}
	cout << ans << endl;
}