#include <stdio.h>
#include <stdlib.h>

int main()
{
    char s[64000], s1[2];
    int i, n, changed = 0;

    scanf("%d", &n);
    gets(s1);
    gets(s);

    for (i = 2; i < 2*n; i+=2)
    {
        if ((s[i] == '1') && (s[i-2]=='1'))
        {
            s[i] = '0';
            changed += 1;
        }
    }
    printf("%d", changed);
    return 0;
}