Codeforces Round #582 (Div. 3) C. Book Reading

#include<bits/stdc++.h>
using namespace std;
#define sf scanf
#define pf printf
#define pb push_back
#define all(a) a.begin(),a.end()
#define i64 long long int
const double pi = 2*acos(0) ;
const int maxn = (i64)2e5+5;

int main()
{
    int q;
    cin>>q;
    i64 n,m;
   while(q--)
    {
        cin>>n>>m;
        if(m==n)
        {
            printf("%d\n",n%10);
            continue;
        }
        else if(m%10 == 0 || m>n)
        {
            printf("0\n");
            continue;
        }
        i64 k = n/m,i;
        i64 sum = 0;
        for(i = 1 ; i<=9 ; i++)
        {
            sum += m*i%10;
        }
        sum = sum*(k/10);
        for(i = 1 ; i<=k%10 ; i++)
        {
            sum += m*i%10;
        }
        cout<<sum<<"\n";
    }
return 0;
}