#include <stdio.h>

int main() {
    int n;
    scanf("%d", &n);

    int arr[n];
    for (int i = 0; i < n; i++) {
        scanf("%d", &arr[i]);
    }
 
    if (arr[0] % 2 != 0) {
        printf("-1\n"); 
    } else {
        int count = 1; 

       
        for (int i = 1; i < n; i++) {
            if (arr[i] % 2 != 0 && arr[i - 1] % 2 != 0) {
                continue;
            }
            count++;
        }

        printf("%d\n", count); 
    }

    return 0;
}