#include <stdio.h>
#include <ctype.h>
int calculateAsciiSum(int n,char *initials) {
    int sum = 0;
for (int i = 0;i < n; i++) {
    if (!salpha(*(initials + i))) {
        return-1;
    }
    sum += *(intials + i);
}
return sum;
}
int main() {
    int n;
    scanf("%d", &n);
    getchar();
    char initials[n]
    scanf("%s", initials);
    int result = calulateAsciiSum(n, initial);
    printf("%d", result);
    return 0;
}