#include<stdio.h>
#include<ctype.h>
int main()
{
    int N;
    scanf("%d",&N);
    if(N <= 0)
    {
        printf("-1");
        return 0;
    }
    int oddsum = 0,evensum = 0;
    char ch;
    for(int i = 0; i < N; i++)
    {
        scanf(" %c",&ch);
        if(!isalpha(ch))
        {
            printf("-1");
            return 0;
        }
        if (ascii % 2 == 0)
        evensum += ascii;
        else
        oddsum +=ascii;
        })
    scanf(" %c", &ch);
    {
        evensum +=ascii;
    }
        printf("%d",abs(oddsum-evensum));
        return 0;
}