foo.cc:11:5: error: redefinition of 'int main()'
   11 | int main()
      |     ^~~~
foo.cc:3:5: note: 'int main()' previously defined here
    3 | int main()
      |     ^~~~
foo.cc: In function 'int main()':
foo.cc:30:18: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
   30 |         if(y%4==0&&y%100!=0||y%400==0)
      |            ~~~~~~^~~~~~~~~~

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