#include <stdio.h>
#include <ctype.h>
int main() {
    int n, sum = 0;
    char arr[100];
    char *ptr;
    scanf("%d" , &n);
    for (int i = 0; i < n; i++) {
        scanf(" %c", &ch[i]);
    }
        ptr = ch;
        for (int i = 0; i < n; i++) {
            if (!isalpha(*(ptr + i))) {
                printf("-1");
                return 0;
            }
            sum += *(ptr + i);
        }
        printf("%d", sum);
        return 0;
}