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