#include <iostream>
using namespace std;

int n,v[1000500],nrc,rez;

int main() {
	// your code goes here
	cin>>n;
	for(int i=1; i<=n; ++i) {
		cin>>v[i];
		if(v[i]) ++nrc;
		else {
			rez+=nrc/2;
			nrc=0;
		}
	}
	rez+=nrc/2;
	cout<<rez;
	return 0;
}