/in/foo.cc: In function 'int main()':
/in/foo.cc:35:6: warning: unused variable 'last' [-Wunused-variable]
  int last=0;
      ^~~~
/in/foo.cc:37:28: warning: iteration 19 invokes undefined behavior [-Waggressive-loop-optimizations]
  for(int i=1;i<=20;i++)a[i]=a[i-1]*2+1;
                        ~~~~^~~~~~~~~~~
/in/foo.cc:37:15: note: within this loop
  for(int i=1;i<=20;i++)a[i]=a[i-1]*2+1;
              ~^~~~