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


int main(){
    int n;
    scanf("%d",&n);
    
    if(n <=0 || n > 100){
        printf("-1");
        return 0;
    }
   char ch;
   int sum = 0;
   int valid = 1;
   for(int i = 0; i < n; i++){
       scanf("%c",&ch);
       
   }
        if(!isalpha((unsigned char)ch)){
            valid=0;
        }else{
             sum +=(int)ch;   
        }
}
 if(valid){
 printf("%d",sum);
 }
 else{
 
            printf("-1");

        }
}    
    
    
    
    printf("%d",sum);
    return 0;
}