#include <stdio.h>
#include <ctype.h>
int main(){
    int n;
    char initials[100]; // assuming max length is 100
    scanf("%d" , &n);
    scanf("%d" , initials);
    int sum = 0;
    int valid = 1;
    for (int i = 0; i < n; i++){
        if(isalpha(initials[i])){
            sum += (int)initials[i];
        }else {
            valid = 0;
            break;
        }
    }
}
if (valid){
    printf("%d\n" , sum);
} else {
    printf("-1\n");
}
return 0;
}