foo.cc: In function 'int main()':
foo.cc:6:16: warning: variable 'a1' set but not used [-Wunused-but-set-variable]
  int n,a,b,c,d,a1,b1;
                ^~
foo.cc:6:19: warning: variable 'b1' set but not used [-Wunused-but-set-variable]
  int n,a,b,c,d,a1,b1;
                   ^~
foo.cc:14:4: warning: 'c' may be used uninitialized in this function [-Wmaybe-uninitialized]
    if(d>c)
    ^~