int sum(ll n) { return (n < 10 ? n : n % 10 + sum(n / 10)); }

ll getans(ll n){
    if (n <= 0) return 0;
    int temp = n % 10, k = (temp*(temp + 1)) / 2;
    return k + (temp + 1)*sum(n/10) + 10 * getans(n / 10 - 1) + 45 * (n/10);
}

//getans(b) - getans(a-1) = ANSWER FOR YOUR QUESTION