foo.cc:4:1: error: 'string' does not name a type; did you mean 'stdin'?
 string b[100001];
 ^~~~~~
 stdin
foo.cc: In function 'int main()':
foo.cc:17:5: error: 'cin' was not declared in this scope
     cin >> n >> m;
     ^~~
foo.cc:17:5: note: suggested alternative:
In file included from /usr/include/x86_64-linux-gnu/c++/7/bits/stdc++.h:75:0,
                 from foo.cc:1:
/usr/include/c++/7/iostream:60:18: note:   'std::cin'
   extern istream cin;  /// Linked to standard input
                  ^~~
foo.cc:19:18: error: 'b' was not declared in this scope
   cin >> a[i] >> b[i];
                  ^
foo.cc:22:6: error: 'i' was not declared in this scope
  for(i=0;i<m;i++){
      ^
foo.cc:35:2: error: 'cout' was not declared in this scope
  cout << b[x];
  ^~~~
foo.cc:35:2: note: suggested alternative:
In file included from /usr/include/x86_64-linux-gnu/c++/7/bits/stdc++.h:75:0,
                 from foo.cc:1:
/usr/include/c++/7/iostream:61:18: note:   'std::cout'
   extern ostream cout;  /// Linked to standard output
                  ^~~~
foo.cc:35:10: error: 'b' was not declared in this scope
  cout << b[x];
          ^

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