#include <bits/stdc++.h>
using namespace std;
struct fast_ios { fast_ios(){ cin.tie(0); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
#define FOR(i, begin, end) for(int i=(begin);i<(end);i++)
#define REP(i, n) FOR(i,0,n)
#define IFOR(i, begin, end) for(int i=(end)-1;i>=(begin);i--)
#define IREP(i, n) IFOR(i,0,n)
#define Sort(v) sort(v.begin(), v.end())
#define Reverse(v) reverse(v.begin(), v.end())
#define all(v) v.begin(),v.end()
#define SZ(v) ((int)v.size())
#define Lower_bound(v, x) distance(v.begin(), lower_bound(v.begin(), v.end(), x))
#define Upper_bound(v, x) distance(v.begin(), upper_bound(v.begin(), v.end(), x))
#define Max(a, b) a = max(a, b)
#define Min(a, b) a = min(a, b)
#define bit(n) (1LL<<(n))
#define bit_exist(x, n) ((x >> n) & 1)
#define debug(x) cout << #x << "=" << x << endl;
#define vdebug(v) cout << #v << "=" << endl; REP(i_debug, v.size()){ cout << v[i_debug] << ","; } cout << endl;
#define mdebug(m) cout << #m << "=" << endl; REP(i_debug, m.size()){ REP(j_debug, m[i_debug].size()){ cout << m[i_debug][j_debug] << ","; } cout << endl;}
#define pb push_back
#define f first
#define s second
#define int long long
#define INF 1000000000000000000
template<typename T> istream &operator>>(istream &is, vector<T> &v){ for (auto &x : v) is >> x; return is; }
template<typename T> ostream &operator<<(ostream &os, vector<T> &v){ for(int i = 0; i < v.size(); i++) { cout << v[i]; if(i != v.size() - 1) cout << endl; }; return os; }
template<typename T> void Out(T x) { cout << x << endl; }
template<typename T1, typename T2> void Ans(bool f, T1 y, T2 n) { if(f) Out(y); else Out(n); }

using vec = vector<int>;
using mat = vector<vec>;
using Pii = pair<int, int>;
using PiP = pair<int, Pii>;
using PPi = pair<Pii, int>;
using bools = vector<bool>;
using pairs = vector<Pii>;

//int dx[4] = {1,0,-1,0};
//int dy[4] = {0,1,0,-1};
//char d[4] = {'D','R','U','L'};

const int mod = 1000000007;
//const int mod = 998244353;
//#define Add(x, y) x = (x + (y)) % mod
//#define Mult(x, y) x = (x * (y)) % mod



signed main(){

    int N; cin >> N;
    string s; cin >> s;

    int ans = 0;
    vec v;
    bool z = true;
    int tmp = 0;
    REP(i, N + 1){
        if(i == N || (s[i] == '0' && z)){
            if(tmp) v.pb(tmp);
            if(SZ(v) >= 2){
                //vdebug(v);
                
                int n = SZ(v);
                REP(j, n) assert(v[j] > 0);
                //0使わない 1 1残し 2 1消費　3 全消し
                mat dp(n, vec(4, -INF));
                dp[0][0] = 0;
                FOR(j, 1, n){

                    REP(k, 4) Max(dp[j][0], dp[j - 1][k]);
                    REP(k, 3) Max(dp[j][3], dp[j - 1][k] + v[j]);
                    
                    if(v[j] >= 2){
                        REP(k, 3) Max(dp[j][1], dp[j - 1][k] + v[j] - 1);

                        Max(dp[j][2], dp[j - 1][0] + v[j - 1]);
                        Max(dp[j][2], dp[j - 1][1] + 1);
                        Max(dp[j][2], dp[j - 1][2] + v[j - 1] - 1);
                    }else{
                        REP(k, 3) Max(dp[j][3], dp[j - 1][k] + v[j] - 1);

                        Max(dp[j][3], dp[j - 1][0] + v[j - 1]);
                        Max(dp[j][3], dp[j - 1][1] + 1);
                        Max(dp[j][3], dp[j - 1][2] + v[j - 1] - 1);
                    }

                    int m = 0;
                    REP(k, 4) Max(m, dp[n - 1][k]);
                    //debug(m);
                    ans += m;

                }
            }
            v = vec(0);
        }else if(s[i] == '0' && !z){
            v.pb(tmp);
            tmp = 0;
            z = true;
        }else if(s[i] == '1'){
            tmp++;
            z = false;
        }
    }
    Out(ans);

    return 0;
}


Measured memory_bytes: 1172