#include <bits/stdc++.h>  // clang-format off
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define each(x,v) for(auto& x : v)
#define all(v) (v).begin(),(v).end()
#define sz(v) ((int)(v).size())
#define ini(...) int __VA_ARGS__; in(__VA_ARGS__)
#define inl(...) long long __VA_ARGS__; in(__VA_ARGS__)
#define ins(...) string __VA_ARGS__; in(__VA_ARGS__)

#ifdef ONLINE_JUDGE
  #define rep(i,N) for(int i = 0; i < (int)(N); i++)
  #define repr(i,N) for(int i = (int)(N) - 1; i >= 0; i--)
  #define rep1(i,N) for(int i = 1; i <= (int)(N) ; i++)
  #define repr1(i,N) for(int i = (N) ; (int)(i) > 0 ; i--)
#else
  #define rep(i,N) for(long long i = 0; i < (long long)(N); i++)
  #define repr(i,N) for(long long i = (long long)(N) - 1; i >= 0; i--)
  #define rep1(i,N) for(long long i = 1; i <= (long long)(N) ; i++)
  #define repr1(i,N) for(long long i = (N) ; (long long)(i) > 0 ; i--)
#endif

using namespace std; void solve();
using ll = long long; template<class T = ll> using V = vector<T>;
using vi = V<int>; using vl = V<>; using vvi = V< V<int> >;
constexpr int inf = 1001001001; constexpr ll infLL = (1LL << 61) - 1;
struct IoSetupNya {IoSetupNya() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); cerr << fixed << setprecision(7);} } iosetupnya;
template<typename T, typename U> inline bool amin(T &x, U y) { return (y < x) ? (x = y, true) : false; }
template<typename T, typename U> inline bool amax(T &x, U y) { return (x < y) ? (x = y, true) : false; }
template<typename T, typename U> ostream& operator <<(ostream& os, const pair<T, U> &p) { os << p.first << " " << p.second; return os; }
template<typename T, typename U> istream& operator >>(istream& is, pair<T, U> &p) { is >> p.first >> p.second; return is; }
template<typename T> ostream& operator <<(ostream& os, const vector<T> &v) { int s = (int)v.size(); for(int i=0;i<s;i++) os << (i ? " " : "") << v[i]; return os; }
template<typename T> istream& operator >>(istream& is, vector<T> &v) { for(auto &x : v) is >> x; return is; }
void in(){} template <typename T,class... U> void in(T &t,U &...u){ cin >> t; in(u...);}
void out(){cout << "\n";} template <typename T,class... U> void out(const T &t,const U &...u){ cout << t; if(sizeof...(u)) cout << " "; out(u...);}
template<typename T>void die(T x){out(x); exit(0);}

#ifdef NyaanDebug
  #include "NyaanDebug.h"
  #define trc(...) do { cerr << #__VA_ARGS__ << " = "; dbg_out(__VA_ARGS__);} while(0)
  #define trca(v,N) do { cerr << #v << " = "; array_out(v , N);} while(0)
  #define trcc(v) do { cerr << "name : " << #v << "\n"; int cnt = 0; each(x , v){cerr << (cnt++) << " : "; trc(x); } } while(0)
#else
  #define trc(...)
  #define trca(...)
  #define trcc(...)
  int main(){solve();}
#endif

#define inc(...) char __VA_ARGS__; in(__VA_ARGS__)
#define in2(s,t)     rep(i,sz(s)){in(s[i] , t[i]);}
#define in3(s,t,u)   rep(i,sz(s)){in(s[i] , t[i] , u[i]);}
#define in4(s,t,u,v) rep(i,sz(s)){in(s[i] , t[i] , u[i] , v[i]);}
using vd = V<double>; using vs = V<string>; using vvl = V< V<> >;
template<typename T,typename U>ll ceil(T a,U b){return (a + b - 1) / b;}

using P = pair<ll,ll>; using vp = V<P>;
constexpr int MOD = /**/ 1000000007; //*/ 998244353;
// clang-format on
///////////////////////////////////////////////////////////


vector<pair<char,int>> RunLengthEncoding(string& S){
  vector<pair<char,int>> ret;
  char c = -1;
  int n = 0;
  for(char x : S){
    if(c == x) n++;
    else{
      if(n) ret.emplace_back(c , n);
      c = x , n = 1;
    }
  }
  if(n) ret.emplace_back(c , n);
  return ret;
}
 
void solve(){
  ini(N);
  ins(S);
  auto rle = RunLengthEncoding(S);
 
  auto f = [&](vi v){
    if(sz(v) <= 1) return 0;
    trc(v);
    vector<vi> dp(sz(v),vi(4,-inf));
    dp[0][0] = 0;
    // 0 何もしない
    // 1 前側を全部使う
    // 2 後ろ側を1個だけ残す
    // 3 後ろ側を全部使う
    rep(i , sz(v) - 1) rep(j , 4) rep(k , 4){
      if(k == 0)
        amax(dp[i + 1][k] , dp[i][j]);
      if(k == 1){
        if(j == 0){
          amax(dp[i + 1][k] , dp[i][j] + v[i]);
        }
        if(j == 1){
          if(v[i] == 1) continue;
          amax(dp[i + 1][k] , dp[i][j] + v[i] - 1);
        }
        if(j == 2){
          if(v[i] == 1) continue;
          amax(dp[i + 1][k] , dp[i][j] + 1);
        }
      }
      if(k == 2){
        if(j != 0 && v[i] == 1) continue;
        if(j == 3) continue;
        amax(dp[i + 1][k] , dp[i][j] + v[i + 1] - 1);
      }
      if(k == 3){
        if(j != 0 && v[i] == 1) continue;
        if(j == 3) continue;
        amax(dp[i + 1][k] , dp[i][j] + v[i + 1]);
      }
    }
    rep(i,sz(v)) trc(dp[i]);
    return *max_element(all(dp[sz(v)-1]));
  };
 
  int ans = 0;
  vi memo;
  each(x , rle){
    if(x.first == '1') memo.push_back(x.second);
    else{
      if(x.second == 1) continue;
      //trc(memo);
      ans += f(memo);
      memo.clear();
    }
  }
  trc(memo);
  ans += f(memo);
  out(ans);
}

Measured memory_bytes: 3352