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

int main() {
    int n;
    scanf("%d", &n);
    
    char initials[n];
    scanf("[^\n]", initials);
    
    int sum = 0;
    char *ptr = initials;
    int invalid = 0;
    
    char *token = initials;
    while (*token != '\0') {
        if (*token != ' ') {
            if (isalpha(*token)) {
                sum += *token;
            } else {
                invalid = 1;
                break;
            }
        }
        token++;
    }
    
    if (invalid) {
        printf("-1\n");
    } else {
        printf("%d\n", sum);
    }
    return 0;
}