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

int main(void){
    int n;
    if(scanf("%d",&n)!=1) return 0;
    
    if(n<1 || n>100){
        printf("-1\n");
        return 0;
    }
    
    char arr[101];
    
    char *p=arr;
    int i;
    int sum=0;
    
    for(i=0;i<n;++i){
        if(scanf("%c",p+i)!=1){
            printf("-1\n");
            return 0;
        }
    }
    
    
    for(i=0;i<n;++i){
        unsignes char uc =(unsigned char)*(p+i);
        if(!isalpha(uc)){
            printf("-1\n");
            return 0;
        }
        sum+=(int) uc;
        
    }
    printf("%d\n",sum);
    return 0;
}