View Compilation Error

0_0_30870731_11066.cpp: In function 'int main()':
0_0_30870731_11066.cpp:10:14: error: 'a' was not declared in this scope
   cout<<(pow(a,3)*(pow(a,3)-1)/2)-(pow(a,3)*3-pow(a,2)*3)<<endl;
              ^