#include <cstdio>
#include <algorithm>
using namespace std;

int N, v[32002], ans, cont, prev = 0;

int main(){

scanf("%d", &N);

for(int i = 1; i <= N; i++){
    scanf("%d", &v[i]);
}

for(int i = 1; i <= N + 1; i++){
    if(v[i] == 0){
        ans += cont / 2;
        cont = 0;
    }
    else cont++;
}

printf("%d\n", ans);

return 0;
}