task 12

#include <iostream>
#include <string>
#include <cctype>

using namespace std;

int main()
{
    string str;
    char a;
    cout << "Enter string: ";
    getline(cin, str);
    cout << "Enter letter: ";
    cin >> a;

    for (int i = 0; i < str.length(); i++) {
        if (str[i] == toupper(a) || str[i] == tolower(a)) {
            str[i] = '@';
        }
    }
    cout << "New string: ";
    cout << str;


    return 0;
}