foo.cc: In function ‘int main()’:
foo.cc:17:18: warning: ‘d’ may be used uninitialized [-Wmaybe-uninitialized]
   17 |                 d+=c;
      |                 ~^~~
foo.cc:6:25: note: ‘d’ was declared here
    6 |         int n,a=1,b=1,c,d;
      |                         ^