Help with this problem

[seeplus]

and what is your current effort to code this? How far have you got? What issues are you having? Post your current code.

[seeplus]

Based upon code from https://cplusplus.com/forum/lounge/284112/

#include <iostream>
#include <vector>
#include <fstream>
#include <algorithm>
#include <iterator>
#include <cctype>
#include <ranges>

namespace rngs = std::ranges;

auto get_words(std::istream& ifs) {
	static constexpr char split { '-' };
	std::vector<std::string> words;

	for (std::string wrd; ifs >> wrd; ) {
		std::string w;

		rngs::copy_if(wrd, std::back_inserter(w), [](unsigned char ch) {return ch == split || std::isalpha(ch); });

		for (const auto& word : rngs::split_view(w, split))
			if (word.begin() != word.end())
				words.emplace_back(word.begin(), word.end());
	}

	return words;
}

int main() {
	std::ifstream ifs("test.txt");

	if (!ifs)
		return (std::cout << "Cannot open file\n"), 1;

	constexpr size_t NoLet { 26 };
	unsigned cnt[NoLet] {};

	std::cout << "Starting letter count:\n";

	for (const auto& w : get_words(ifs))
		++cnt[static_cast<char>(std::toupper(static_cast<unsigned char>(w[0]))) - 'A'];

	for (size_t l {}; l < NoLet; ++l)
		if (cnt[l])
			std::cout << static_cast<char>('A' + l) << ' ' << cnt[l] << '\n';
}

Starting letter count:
A 4
B 2
D 1
G 2
I 1
M 2
O 1
R 1
S 1
T 6
W 2

[seeplus]

where i can post the code?

Here. Use code tags when posting code:

[code]
formatted code goes here
[/code]

[seeplus]

... but can you understand and explain it?

If that was for a homework exercise, then...