#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
int main() {
    int n;
    scanf("%d",&n);
    char ch;
    int odd sum = 0, even sum = 0;
    for (int i = 1; i <= n; i++) {
        scanf(" %c",&ch);
        if (!isalpha(ch)) {
            printf("-1\n");
            return 0;
        }
        if (i % 2 == 0) 
            even sum += (int)ch;
        else            
            odd sum += (int)ch;
    }
    printf("%d\n", abs(odd sum - even sum));
    return 0;
}