#include <cstdio>
#include <iostream>
#include <fstream>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <bitset>
#include <algorithm>
#include <sstream>
#include <iomanip>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <cstring>
#include <string>
#include <ctime>
#include <cassert>
#include <utility>

using namespace std;

#define MAXN 32005

int A[MAXN];
int N;

int main() {
//	freopen("date.in", "r", stdin);
//	freopen("date.out","w", stdout);
	
	scanf("%d", &N);
	for(int i = 0; i < N; i++)
        scanf("%d", &A[i]);
	
	int ans = 0;
	
	int i = 0;
	while(i < N) {
	    if(A[i] == 0) {
	        i++;
            continue;
	    }
	    
        int j = i;
        while(j + 1 < N && A[j + 1] == 1)
            j++;
        
        ans += (j - i + 1) / 2;
        i = j + 1;
	}
	
	printf("%d\n", ans);
	
	return 0;
}