codeforces 817C Really Big Numbers

第一眼感觉是dp，但是不知道该怎么写。看cf题解是二分，就按照题解写了，不过还是要看看dp解法。

#include <bits/stdc++.h>using namespace std;typedef long long ll;ll n,s;bool check(ll num){    ll sum = 0;    ll temp = num;    while(temp)    {        sum += temp%10;        temp/=10;    }    return abs(num-sum) >= s;}int main(){    cin >> n >> s;    ll l = 1;    ll r = n;    ll mid = (l+r) >> 1;    while(l <= r)    {        if(check(mid))            r = mid-1;        else            l = mid+1;        mid = (l+r) >> 1;    }    cout << n-l+1 << endl;    return 0;}