#include <iostream>
#include <algorithm>
#include <functional>
#include <cmath>
#include <vector>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <queue>
#include <ctime>
#include <cassert>
#include <complex>
#include <string>
#include <cstring>
#include <chrono>
#include <random>
#include <bitset>
#include <array>
#include<utility>
#include<stack>
#include <iomanip>
//#include<atcoder/all>
//using namespace atcoder;
using namespace std;
#define P pair<int,int>
#define Graph vector<vector<int>>
#define float long double
#define rep(i,a,b) for(int i=(a);i<(b);++i)
#define repi(itr,m) for(auto itr=(m).begin();itr!=(m).end();itr++)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define vi vector<int>
#define int long long
const int INF=1e18;
int dx[8]={0,1,0,-1,-1,-1,1,1};
int dy[8]={1,0,-1,0,-1,1,-1,1};
template<typename T> inline bool chmax(T &a, T b) { return ((a < b) ? (a = b, true) : (false));}
template<typename T> inline bool chmin(T &a, T b) { return ((a > b) ? (a = b, true) : (false));}
struct point{
  int x,y;
};
void yn(bool t){
  if(t)cout<<"Yes"<<endl;
  else cout<<"No"<<endl;
}

int solve(){
  int n;cin>>n;
  vector<int>s(n+1,1);
  rep(i,1,n+1){
    char c;cin>>c;
    s[i]=c-'0';
  }
  vector<int>yes(n+1,0);
  vector<int>no(n+1,0);
  vector<int>val;
  int idx=0;
  rep(i,1,n+1){
    if(s[i]==0){
      val.push_back(i);
      if(val.size()==1)idx=i;
    }
  }
  yes[idx]=1;
  
  rep(i,idx+1,n+1){
    if(s[i]==1){
      yes[i]=yes[i-1];
      no[i]=no[i-1];
    }
    else{
      auto itr=lower_bound(all(val),i);
      itr--;
      idx=*itr;
      if(i-idx>=3){
        yes[i]=min(yes[i-1],no[i-1])+1;
        no[i]=no[i-1];
      }
      else{
        yes[i]=min(yes[i-1],no[i-1])+1;
        no[i]=yes[i-1];
      }
    }
  }
  
  cout<<min(yes[n],no[n])<<endl;
  assert(true);
  return 0;
}

signed main(){
  ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  solve();
}