foo.cc: In function 'int main()':
foo.cc:14:8: warning: 'char* gets(char*)' is deprecated [-Wdeprecated-declarations]
   14 |  gets(a);
      |        ^
In file included from /usr/include/stdio.h:867,
                 from /usr/include/c++/9/cstdio:42,
                 from /usr/include/c++/9/ext/string_conversions.h:43,
                 from /usr/include/c++/9/bits/basic_string.h:6493,
                 from /usr/include/c++/9/string:55,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:2:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:235:1: note: declared here
  235 | gets (char *__str)
      | ^~~~
foo.cc:14:8: warning: 'char* gets(char*)' is deprecated [-Wdeprecated-declarations]
   14 |  gets(a);
      |        ^
In file included from /usr/include/stdio.h:867,
                 from /usr/include/c++/9/cstdio:42,
                 from /usr/include/c++/9/ext/string_conversions.h:43,
                 from /usr/include/c++/9/bits/basic_string.h:6493,
                 from /usr/include/c++/9/string:55,
                 from /usr/include/c++/9/bits/locale_classes.h:40,
                 from /usr/include/c++/9/bits/ios_base.h:41,
                 from /usr/include/c++/9/ios:42,
                 from /usr/include/c++/9/ostream:38,
                 from /usr/include/c++/9/iostream:39,
                 from foo.cc:2:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:235:1: note: declared here
  235 | gets (char *__str)
      | ^~~~
foo.cc:10:12: warning: unused variable 'N' [-Wunused-variable]
   10 |  int const N=100;
      |            ^
foo.cc:11:8: warning: unused variable 'j' [-Wunused-variable]
   11 |  int i,j,n,s1,s2,s3,s4;
      |        ^
foo.cc:14:6: warning: ignoring return value of 'char* gets(char*)', declared with attribute warn_unused_result [-Wunused-result]
   14 |  gets(a);
      |  ~~~~^~~
/usr/bin/ld: /tmp/cc748bEN.o: in function `main':
foo.cc:(.text.startup+0x2a): warning: the `gets' function is dangerous and should not be used.

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