4.数位和为K的个数

#include<iostream>
using namespace std;
int main() {
int l, r, k, sum = 0;
cin >> l >> r >> k;
for (int i = l; i <= r; i++) {
int num = i;
int s = 0;
while (num > 0) {
s += num % 10;
num /= 10;
}
if (s == k) {
sum++;
}
}
cout << sum << endl;
return 0;
}