#include<stdio.h>
#include<ctype.h>
int main() {
    int N;
    scanf("%d",&N);
    
    if(N <= 0) {
        printf("-1");
        return 0;
    }
    char ch;
    int oddSum=0;
evenSum=0;
    for(int i=0;i<N;i++) {
        scanf("%c",&ch);
        if(isalpha(ch)) {
            printf("-1");
            return 0;
        }
    
    int ascii = (int)ch;
    if(ascii % 2 == 0) {
        evenSum +=ascii;
        
    }
    else {
        oddSum += ascii;
    }
    }
    printf("%d",oddSum-evenSum);
    return 0;
}