CF817C Really Big Numbers

思路：

二分。

实现：

 #include <iostream>
 #include <cstdio>
 using namespace std;

 typedef long long ll;

 const ll MAXN = 1e18;

 ll n, s;

 bool check(ll x)
 {
     ll sum = ;
     ll tmp = x;
     while (x)
     {
         sum += x % ;
         x /= ;
     }
     return tmp - sum >= s;
 }

 int main()
 {
     cin >> n >> s;
     ll l = , r = MAXN + , res = MAXN + ;
     while (l <= r)
     {
         ll m = (l + r) >> ;
         if (check(m)) r = m - , res = m;
         else l = m + ;
     }
     cout << (n >= res ? n - res +  : ) << endl;
     return ;
 }