var n,i,c:integer;
    x,y:0..1;

begin

 readln(n); c:=0;

 i:=2; read(x);

 while i<=n do
       begin
        i:=i+1;
        read(y);
        if (x=1) and (y=1) then begin
                                c:=c+1;
                                y:=0;
                                end;
        x:=y;
       end;

 writeln(c);
end.