#include <bits/stdc++.h>
using namespace std;
#define oo 1000000010
#define mod 1000000007
const int N = 300010;

int n , arr[N] , cur[N];

vector< pair< int , int > > v;


int main(){
	scanf("%d",&n);
	int last = 0;
	for(int i = 0 ;i < n - 1;i++){
		scanf("%d",&arr[i]);
		cur[i] += arr[i];
		cur[i + 1] += arr[i];
		if(arr[i] == 0){
			v.push_back(make_pair(last , i));
			last = i + 1;
		}
	}
	v.push_back(make_pair(last , n - 1));
	int ans = 0;

	for(int a , b , i = 0 ;i < (int)v.size();i++){
		a = b = 0;
		for(int j = v[i].first ;j <= v[i].second;j++){
			if(cur[j] != 0)
				b = 1;
			if(cur[j] & 1){
				a++;
			}
		}
		ans += max(b , a / 2);
	}
	cout << max(0 , ans - 1) << endl;

	return 0;
}