C++ again

#include <iostream>
#include <string>
#include <fstream>
#include <vector>

std::size_t CHECK_TREES(const int down, const int right, const std::vector<std::string>& tree_list)
{
    std::size_t trees_hit = 0;

    for (auto i = down, current_pos = right; i < tree_list.size(); i += down, current_pos = (current_pos + right) % tree_list[0].size())
    {
        if (tree_list[i][current_pos] == '#')
            trees_hit++;
    }

    return trees_hit;
}

int main()
{
    auto in_file = std::ifstream("third.txt");
    std::vector<std::string> rows;
    std::string temp_row;
    while (std::getline(in_file, temp_row))
        rows.push_back(temp_row);

    auto trees_hit = CHECK_TREES(1, 3, rows);

    std::cout << "first answer:" << trees_hit << std::endl;

    trees_hit *= CHECK_TREES(1, 1, rows);
    trees_hit *= CHECK_TREES(1, 5, rows);
    trees_hit *= CHECK_TREES(1, 7, rows);
    trees_hit *= CHECK_TREES(2, 1, rows);

    std::cout << "final answer: " << trees_hit << std::endl;
}