Using Javascript

Question

<asp:Button ID="btn" runat="server" Text="C" OnClientClick="Check()" />

        <input type="checkbox" id="chk1" />
        <input type="checkbox" id="ch2" />
        <input type="checkbox" id="chk3" />
        <input type="checkbox" id="chk4" />
On clicking the button , i want to find the number of checkboxes selected .

Using javascript?