#include<stdio.h>
int main(){
int lower_bound, upper_bound;
scanf("%d %d", &lower_bound, &upper_bound);
if (lower_bound < 0 || upper_bound < 0 ||lower_bound > upper_bound){
    print("-1");
    return 0;
}
int sum_of_ages = 0;
for (int age = lower_bound; age <= upper_bound; age++){
    if (age % 7 == 0){
        sum_of_ages += age;
    }
}
printf("%d\n", sum_of_ages);
return 0;
}