output Integer to stdout

[Ashraf AbuSharekh]

hi
i just begun using crypto++, i am trying to  print Integer to the screen 
using the operator << and get this error
error C2678: binary '<<' : no operator defined which takes a left-hand 
operand of type 'class ostream_withassign' (or there is no acceptable 
conversion)
the code

#include <iostream.h>
#include <integer.h>
int main()
{
        CryptoPP::Integer a(10), b(20),c ;
        c = a + b ;
        cout << c << endl;
        return 0;
}
if anyone could tell me how to output integer to stdout
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