visual studio c++ error link1120

I was writing a C++ program in visual studio 2013 and the
program is very simple one and when I compile there was error
this is third time I see this error.
Here's the program:

#include 
using namespace std;

int main()

{
    int x, y, z = 0;
    char c;
    bool quit = true;

    while (quit == true)
    {
        cout << "enter a number : ";
        cin >> x;
        cout << "enter a sign : ";

        cin >> c;
        cout << "enter another number : ";
        cin >> y;

        if (c == '+')
            z = x + y;
        if (c == '-')
            z = x - y;
        if (c == '*')
            z = x * y;

        if (c == '/' && y != 0)
            z = x / y;
        if (c == 'q')
            quit = false;

        cout << "Answer is : " << z << endl;
    }

    return 0;
}

...and here's the error:

1>------ Build started: Project: Calculator_madeByMe, Configuration: Debug Win32 ------
1>  Source.cpp
1>MSVCRTD.lib(crtexew.obj) : error LNK2019: unresolved external symbol _WinMain@16 referenced in function ___tmainCRTStartup
1>C:\Users\Sorena\documents\visual studio 2013\Projects\Calculator_madeByMe\Debug\Calculator_madeByMe.exe : fatal error LNK1120: 1 unresolved externals
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========>