CF817C.Really Big Numbers(二分 思维)

// Problem: C. Really Big Numbers
// Contest: Codeforces - Educational Codeforces Round 23
// URL: https://codeforces.com/problemset/problem/817/C
// Memory Limit: 256 MB
// Time Limit: 1000 ms
// 
// Powered by CP Editor (https://cpeditor.org)

#include<bits/stdc++.h>

using namespace std;

typedef long long ll;

ll n,s;

bool check(ll x){
  ll sum=0,tmp=x;
  while(x){
    sum+=x%10;x/=10;
  }
  return abs(sum-tmp)>=s;
}

int main(){
  cin>>n>>s;
  
  ll l=1,r=2e18,ans=n;
  
  //cout<<check(11)<<" "<<check(12)<<" "<<check(13)<<"\n";
  
  while(l<=r){
    ll mid=(l+r)/2;
    if(check(mid)) ans=mid,r=mid-1;
    else l=mid+1;
  }
  cout<<max(0ll,n-ans+1)<<endl;
  return 0;
}