#include<stdio.h>
    long long calculate_age_sum(int upper, int lower);
    long long sum=0;
    for(int age = lower; age <= upper; age++){
        if(age%7 == 0){
            sum+= age;
        }
        return sum;
    }
    int main()
    printf("Enter age range(lower upper):",range(lower upper));
    if(scanf("%d %d", &upper, &lower)!=2)
    {
        printf("Invalid Input\n");
        return 1;
    }
    if(lower < 0||upper < 0||lower > upper)
    {
        printf("Invalid Input\n");
    }
    else
    {
        long long sum = calculate_age_sum(lower , upper);
        printf("Sum of ages: %lld\n",sum);
    }
    return 0;
}