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  vanh14  đã bình luận lúc 16, Tháng 7, 2025, 16:01

  include <iostream>

  include <algorithm>

  include <map>

  include<unordered_map>

  include <set>

  include <fstream>

  include <vector>

  include <math.h>

  using namespace std;

  define ll long long

  int main() {

  int n; ll tong = 0; cin >> n;
  for (int i = 7; i <= n; i += 7) {
      tong += i;
  }
  cout << tong;
  return 0;
  

  }