Given the following method:

public static int f$($String input$)\{$

Stack stack $=$ new Stack$()$;

int n $=0$;

for $($int i $=0$; i $<$ input.length$()$; i$++)\{$

if $($input$.$charAt$($i$)==\text{'}$d$\text{'})$

stack.push$(\text{'}$d$\text{'})$;

else if $($input$.$charAt$($i$)==\text{'}$b$\text{'})\{$

if $(!$stack.empty$()$ && stack.pop$()==\text{'}$d$\text{'})$ n$++$;

$\}$

$\}$

return n;

$\}$