View Compilation Error

0_0_37712396_19611.cpp:7:11: error: expected initializer before 'a'
 long logn a[N];
           ^
0_0_37712396_19611.cpp: In function 'int main()':
0_0_37712396_19611.cpp:17:8: error: 'a' was not declared in this scope
   cin>>a[i];
        ^
0_0_37712396_19611.cpp:19:3: error: 'a' was not declared in this scope
   a[i]+=a[i+1]/2;
   ^
0_0_37712396_19611.cpp:20:6: error: 'a' was not declared in this scope
   if(a[0])
      ^