###GolfScript, 107 characters###

    .n?):L;'1'/{,}%{1$+)}*;][]\{:A{{+}+[1L.~)-1]%&}+1$\,.@^\[[[A]]+{|}*]+}/{.{L%}{%$..&1$,1$,/*$=}:C~\{L/}C&},,

The input must be given on STDIN. 

Examples:

    11
    01
    -
    0
    
    111
    111
    -
    1
    
    100
    001
    001
    -
    2
    
    11100
    10101
    11100
    -
    1
    
    101
    010
    101
    -
    5