error: undefined reference

#include<windows.h>
 
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
 
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
                    LPSTR lpCmdLine, int nCmdShow)
{
    HWND hMainWnd;
    char szClassName[] = "MyClass";
 
    MSG msg;
    WNDCLASSEX wc;
 
    wc.cbSize = sizeof(wc);
    wc.style = CS_HREDRAW | CS_VREDRAW;
    wc.lpfnWndProc = WndProc;
    wc.cbClsExtra = 0;
    wc.cbWndExtra = 0;
    wc.hInstance = hInstance;
    wc.hIcon = LoadIcon(NULL, IDI_QUESTION);
    wc.hCursor = LoadCursor(NULL, IDC_UPARROW);
    wc.hbrBackground = /*(HBRUSH)GetStockObject(WHITE_BRUSH)*/(HBRUSH)(COLOR_WINDOW+11);
    wc.lpszMenuName = NULL;
    wc.lpszClassName = szClassName;
    wc.hIconSm = LoadIcon(NULL, IDI_ERROR);
 
    if(!RegisterClassEx(&wc)){
        MessageBox(NULL, "Cannot register class", "Error", MB_OK);
        return 0;
    }
 
    hMainWnd = CreateWindow(
        szClassName, "A Hello Application", WS_OVERLAPPEDWINDOW,
        0, 0, 500, 400,
        (HWND)NULL, (HMENU)NULL,
        (HINSTANCE)hInstance, NULL
    );
 
    if(!hMainWnd){
        MessageBox(NULL, "Cannot create main window", "Error", MB_OK);
        return 0;
    }
 
    ShowWindow(hMainWnd, nCmdShow);
    UpdateWindow(hMainWnd);
 
    while(GetMessage(&msg, NULL, 0, 0)){
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
 
    return msg.wParam;
}
 
LRESULT CALLBACK WndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    HDC hDC;
    PAINTSTRUCT ps;
    RECT rect;
 
    int userReply;
 
    switch(uMsg)
    {
        case WM_PAINT:
            hDC = BeginPaint(hWnd, &ps);
 
            GetClientRect(hWnd, &rect);
            DrawText(hDC, "Hello, world!", -1, &rect, DT_SINGLELINE | DT_CENTER | DT_VCENTER);
            {
                POINT pt[5] = {{100, 100}, {200, 100}, {200, 200}, {100, 200}, {100, 100}};
                MoveToEx(hDC, pt[0].x, pt[0].y, NULL);
                for (int i = 0; i < 5; ++i)
                    LineTo(hDC, pt[i].x, pt[i].y);
            }
            EndPaint(hWnd, &ps);
            break;
        case WM_CLOSE:
            userReply = MessageBox(hWnd, "Захуярить приложение?", "", MB_YESNO | MB_ICONQUESTION);
            if(IDYES == userReply)
                DestroyWindow(hWnd);
            break;
        case WM_DESTROY:
            PostQuitMessage(0);
            break;
        default:
            return DefWindowProc(hWnd, uMsg, wParam, lParam);
    }
 
    return 0;
}