/in/foo.cc: In function 'int main()':
/in/foo.cc:34:8: warning: 'x4' may be used uninitialized in this function [-Wmaybe-uninitialized]
   b=(x3+x4)/2;
     ~~~^~~~
/in/foo.cc:34:8: warning: 'x3' may be used uninitialized in this function [-Wmaybe-uninitialized]
/in/foo.cc:30:8: warning: 'x2' may be used uninitialized in this function [-Wmaybe-uninitialized]
   a=(x1+x2)/2;
     ~~~^~~~
/in/foo.cc:30:8: warning: 'x1' may be used uninitialized in this function [-Wmaybe-uninitialized]