Основы С++

#include <iostream>
#include <iomanip>

using namespace std;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    cout << 1234 << '\n';
    cout << 1234 << '\n';
    cout << 1234 << '\n';
    cout << 1234 << '\n';
    cout << 1234 << '\n';

    cout.flush();
    cout << 1234 << endl;

    cout << 1234 << '\n';
    int x;
    cin >> x;

    int x1; // +- 2*10^9
    long long x2; // +- 9*10^18
    double x3;
    char x4 = '1';

    unsigned int x5; // 0..4*10^9
    unsigned long long x6; // 0..18*10^18
    size_t x7; // IB 64-бит

    cout << (long long)x1 << ' ' << int(x2) << ' ' << (long long)(x1) << ' ' << static_cast<long long>(x1);

    cout << x1 + x2 << '\n';
    cout << x2 + x3 << '\n'; // double

    cout << char(x4 + x4) << '\n';
    cout << int(x4) << ' ' << char(123) << '\n';

    cout << x1 + x5 << '\n'; // unsigned int

    vector<int> v = {1, 2};
    cout << 1 - (long long)(v.size()); // -1 unsigned long long


    cout << fixed << setprecision(15) << x3 << '\n';

    bool x5 = false; // true.
    cout << boolalpha << x5; // False

    if (-1) {
        cout << "True";
    }

    cout << 1'000'000'000ll * 1'000'000'000;

    int a = 10;
    cout << ++a << ' ' << a << '\n' << a++ << ' ' << a << '\n'; // 11 11 11 12
    --a;
    a--;
    cout << a--;

    if () {

    } else if () {

    } else if () {

    } else {

    }

    int n;
    cin >> n;
    int i = 20;
    for (i = 0; i < n; ++i, --n) {
        i += 10;
    }
    while (n > 0) {
        int b1 = 10;
        --n;
    }
    {
        {
            int b1; // мусор
            int sum = 0;
            for (int i = 0; i < n; ++i) {
                sum += i;
            }
            cout << sum;
        }
    }
}