View Compilation Error

0_0_38338284_32380.cpp: In function 'int main()':
0_0_38338284_32380.cpp:12:3: error: 's' was not declared in this scope
   s=0;
   ^
0_0_38338284_32380.cpp:18:8: error: 'a' was not declared in this scope
   res=(a[n+1]+n*a[0]-2*s)/(n+1);
        ^