foo.cc:3:9: error: 'cout' does not name a type
    3 |         cout<<"请输入长";
      |         ^~~~
foo.cc:4:9: error: 'cin' does not name a type
    4 |         cin>>a;
      |         ^~~
foo.cc:5:9: error: 's' does not name a type
    5 |         s=a*a;
      |         ^
foo.cc:6:9: error: 'cout' does not name a type
    6 |         cout<<"面积为:"<<s;
      |         ^~~~
foo.cc:7:9: error: expected unqualified-id before 'return'
    7 |         return 0;
      |         ^~~~~~

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