666，不<AC>怎么能发题解呢？

正经题解

#include<bits/stdc++.h>
using namespace std;
int func(int x) {
    int sum = 0;
    while (x != 0) {
        sum += x % 10;
        x /= 10;
    }
    return sum;
}
int main() {
	int a, b;
    cin >> a >> b;
    int sum = 0;
    for (int i = a; i <= b; i++) sum += func(i);
    cout << sum;
    return 0;
}