View Compilation Error

0_0_23260176_16381.cpp: In function 'int main()':
0_0_23260176_16381.cpp:11:5: error: '__' was not declared in this scope
     __
     ^
0_0_23260176_16381.cpp:19:23: error: 'a' was not declared in this scope
     for(i=4;i<51;i++) a[i]=a[i-1]+2*a[i-2];
                       ^
0_0_23260176_16381.cpp:21:36: error: 'a' was not declared in this scope
     while(cin>>i) printf("%I64d\n",a[i]);
                                    ^