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