View Compilation Error

0_0_39583956_11058.cpp:4:11: error: 'mxn' was not declared in this scope
    4 | i64 T,n,a[mxn];
      |           ^~~
0_0_39583956_11058.cpp: In function 'int main()':
0_0_39583956_11058.cpp:18:30: error: 'a' was not declared in this scope
   18 |                         cin>>a[i];
      |                              ^
0_0_39583956_11058.cpp:22:36: error: 'a' was not declared in this scope
   22 |                         ans+=merge(a[0],a[i]);
      |                                    ^