3.27

#include <iostream>
#include <string>
using namespace std;

int main() {
    string firstName, lastName;
    int number;

    cin >> firstName >> lastName >> number;

    string loginName = firstName.substr(0, 6);
    loginName += lastName[0];
    loginName += '_';
    loginName += to_string(number % 10);

    cout << "Your login name: " << loginName << endl;

    return 0;
}