C++ :

#include<bits/stdc++.h>
using namespace std;
long long t,n,s;
int main()
{
//  freopen("1.in","r",stdin);
//  freopen("1.out","w",stdout);
    cin>>t;
    while(t--)
    {
        cin>>n>>s;
        long long ws=0,he=0;
        for(long long i=n;i>0;i/=10)
            ws++,he+=i%10;
        if(he<=s) {cout<<0<<endl;continue;}
        long long shi=1;
        for(long long i=1;i<ws;i++)
            shi*=10;
        long long sum=0,ans=0;
        for(long long i=n,j=shi;j>0;j/=10)
        {
            long long x=i/j%10;
            sum+=x;
            if(sum<s) ans=ans*10+x;
            else{
                if(sum==0){
                    ans=shi*100;
                }
                else{
                    ans++;
                    ans*=j*10;
                }
                cout<<ans-n<<endl;
                break;
            }
        }
    }
    return 0;
}