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- #include<iostream>
- using namespace std;
- int main(){
- long long n, k;
- cin>>n>>k;
- long long cur=0;
- long long tot=0;
- long long size_periodou=0;
- for(long long i=0;i<n;i++){
- if(cur==0 && i!=0) break;
- cur+=(k%10);
- cur%=10;
- tot+=cur;
- size_periodou ++;
- }
- long long num_periodoi = (n/k) / size_periodou;
- long long ans = num_periodoi * tot;
- long long ipolipo = (n/k) % size_periodou;
- for(long long i=1;i<=ipolipo;i++){
- ans+= (i * k)%10;
- }
- cout<<ans<<endl;
- }
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