View Compilation Error

0_0_34927728_25428.cpp:4:8: error: array bound is not an integer constant before ']' token
 int a[N];
        ^
0_0_34927728_25428.cpp: In function 'int main()':
0_0_34927728_25428.cpp:28:45: error: 'a' was not declared in this scope
         for (int i = 0; i < n; ++ i) cin >> a[i];
                                             ^
0_0_34927728_25428.cpp:29:20: error: 'a' was not declared in this scope
         merge_sort(a,0,n-1);
                    ^