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

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

    char initials;
    int count = 0, sum_even = 0, sum_odd = 0;
    int invalid = 0;

    // Use a loop to read 'n' alphabetic initials
    for (int i = 0; i < n; i++) {
        scanf("%s", initials);
        char ch = initials;

        // Check if character is alphabetic
        if (!isalpha(ch)) {
            invalid = 1;
            break;
        }
        int ascii = (int)ch;
        if (ascii % 2 == 0)
            sum_even += ascii;
        else
            sum_odd += ascii;
        count++;
    }

    // Validate count and validity
    if (invalid || count != n) {
        printf("-1\n");
    } else {
        printf("%d\n", abs(sum_even - sum_odd));
    }

    return 0;
}