View Code of Problem 3920

#include<iostream>
#include<vector>
#include<algorithm>
#include<iomanip>
#include<string>
#include<cmath>
#include<unordered_map>
#include<stack>

using namespace std;

int main()
{
	int q;
	cin >> q;

	for (int i = 0; i < q; i++) {

		long long n, m;
		cin >> n >> m;

		long long nums = n / m;	//总数

		vector<int> lowest;

		long long temp = m;
		while (temp <= n) {

			lowest.push_back(temp % 10);	//第一个最后一位

			temp += m;

			if (lowest[0] == temp % 10)
				break;
		}
		
		if (lowest.size() == 0) {

			cout << 0 << endl;
			continue;
		}

		long long sum = 0;

		for (int j = 0; j < lowest.size(); j++)
			sum += lowest[j];

		long long bei = nums / lowest.size();

		sum = sum * bei;

		for (int j = 0; j < nums - bei * lowest.size(); j++)
			sum += lowest[j];

		cout << sum << endl;
	}
}