foo.cc:3:7: warning: ISO C++ forbids declaration of 'main' with no type [-Wreturn-type]
    3 | main ()
      |       ^
foo.cc: In function 'int main()':
foo.cc:5:18: warning: 'sum' may be used uninitialized in this function [-Wmaybe-uninitialized]
    5 |  int i,n,x1,ls=0,sum;
      |                  ^~~

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