Помогите пожалуйста с задачей на c++

#include <iostream>
#include <string>
#include <vector>
using namespace std;
vector<string> lines(const size_t size, const size_t length = 20) {
string word;
vector<string> words;
do {
cout << "word: ";
cin >> word;
const auto len = word.length();
if (len && len <= length) words.emplace_back(word);
} while (words.size() < size);
return words;
}
void print(const vector<string>& words) {
for (const auto &word : words) cout << word << '\n';
cout.put('\n');
}
void del_front(vector<string>& words) {
for (auto &word : words) word.erase(word.begin(), word.begin() + 1);
}
void ins(vector<string>& words, const char letter, const char mark) {
for (auto &word : words) {
const auto pos = word.find(mark);
if (pos != string::npos) word.insert(pos + 1, string() + letter);
}
}
char get_char(const char* msg) {
cin.ignore(numeric_limits<streamsize>::max(), '\n');
cout << msg;
return cin.get();
}
size_t get_size(const char* msg, const size_t max_length = 20, const size_t min_length = 1) {
size_t size;
do {
cout << msg;
cin >> size;
} while (size < min_length || size > max_length);
return size;
}
int main() {
const auto n = get_size("n: ");
auto words = lines(n);
system("cls");
print(words);
del_front(words);
print(words);
const auto mark = get_char("mark: ");
const auto letter = get_char("letter: ");
cout.put('\n');
ins(words, letter, mark);
print(words);
system("pause");
}