/*

*/
#include <stdio.h>

int main(){
	int N, counter, st, pst;//prejsnji st
	pst = 0;
	counter = 0;

	scanf("%i", &N);
	for(int i=0; i<N; i++){
		scanf("%i", &st);
		if(st == 0){
			pst = 0;
		}
		else{//st == 1
			if(pst == 0){
				pst = 1;
			}
			else{//pst == 1
				pst = 0;
				counter++;
			}
		}
	}
	printf("%i\n", counter);
	return 0;
}