售卖数字（二分）找不到原题

#include <iostream>
using namespace std;
typedef long long ll;
ll A,B,X;
bool check(ll mid){
    ll cnt = 0,t = mid;
    while(t){
        t /= 10;
        cnt++;
    }
    if(A * mid + B * cnt <= X) return true;
    else return false;
}
int main(){
    cin >> A >> B >> X;
    ll l = 0,r = 1e9;
    while (l < r){
        ll mid = (l + r + 1) >> 1;
        if(check(mid)) l = mid;
        else r = mid -1 ;
    }
    cout << l << endl;
    return 0;
}