#include <iostream>
#include <fstream>
#include <vector>
#include <string.h>
#include <set>
#include <queue>
#include <iomanip>

#define EPS 0.0000001
#define nMax 100000000
#define pb push_back
#define mkp make_pair
#define pii pair<int, int>
#define pdd pair<double, double>
#define piiii pair<pii, pii>
#define pdddd pair<pdd, pdd>
#define x first
#define y second


using namespace std;

//ifstream fin("001.in");

int main()
{
    int n, bit, nrBit=0, Sol=0;
    cin>>n;
    for(int i=1; i<=n; i++)
    {
        cin>>bit;
        if(bit)
            nrBit++;
        else
        {
            Sol+=nrBit/2;
            nrBit=0;
        }
    }

    Sol+=nrBit/2;
    cout<<Sol;

    return 0;
}