foo.cc: In function 'int main()':
foo.cc:28:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for(int i=0;i<num.size()-1;i+=2)
                         ~^~~~~~~~~~~~~
foo.cc:36:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for(int i=0;i<num1.size()-1;i+=2)
                         ~^~~~~~~~~~~~~~
foo.cc:20:32: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     for(int i=1;i<=n;++i) scanf("%d",&a[i]);
                           ~~~~~^~~~~~~~~~~~

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