foo.cc: In function 'int main()':
foo.cc:13:7: warning: statement has no effect [-Wunused-value]
   days==31+day;
   ~~~~^~~~~~~~
foo.cc:57:28: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
   if(year%400==0||year%4==0&&year%100!=0)
                   ~~~~~~~~~^~~~~~~~~~~~~
foo.cc:62:6: warning: 'days' may be used uninitialized in this function [-Wmaybe-uninitialized]
  cout<<days;
  ~~~~^~~~~~

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