#include<bits/stdc++.h>
using namespace std;

typedef long long int           ll;
typedef unsigned long long int  ull;
typedef vector<int>             vi;
typedef vector<ll>              vll;
typedef vector<vi>              vvi;
typedef pair<int,int>           pii;
typedef vector< pii >           vpii;
typedef set<int>                sti;

#define pb                      push_back
#define pob                     pop_back
#define mp                      make_pair
#define ff                      first
#define ss                      second
#define loop1(i,n)              for(int i=1; i<=int(n); i++)
#define loop(i,y)               for(int i=0; i<int(y); i++)
#define rloop(i,y)              for(int i=int(y); i>0; i--)
#define rloop1(i,y)             for(int i=int(y)-1; i>=0; i--)
#define read()                  freopen("input.txt", "r", stdin)
#define write()                 freopen("output.txt", "w", stdout)
#define cspf(i)                 printf("Case %d: \n", i)
#define all(v)                  v.begin(),v.end()
#define rall(v)                 v.rbegin(),v.rend()
/// Constants
#define eps                     1e-9
#define PI                      acos(-1.0)  // 3.1415926535897932

int main()
{
    int n,k=0;
    cin>>n;
    int a[n+1],flg[n+1];
    memset(flg,0,sizeof(flg));
    loop(i,n)
        cin>>a[i];
    for(int i=1;i<n-1;i++){
        if(a[i]==0){
            if(a[i-1]==1 && a[i+1]==1){
                flg[k]=i;
                k++;
            }
        }
    }
    int j,y,mx=0;
    loop(i,k){
        if(flg[i]==0)
            continue;
        y=flg[i+1]-flg[i];
        if(y==2){
            mx++;
            flg[i]=0;
            flg[i+1]=0;
        }
        else{
            flg[i]=0;
            mx++;
        }
    }
    cout<<mx<<endl;
    return 0;
}