View Compilation Error

0_0_24962000_12097.cpp:3:17: error: 'n' was not declared in this scope
 long long int f(n);
                 ^
0_0_24962000_12097.cpp: In function 'int main()':
0_0_24962000_12097.cpp:13:22: error: 'f' cannot be used as a function
             sum = f(n);
                      ^
0_0_24962000_12097.cpp: In function 'long long int f(int)':
0_0_24962000_12097.cpp:19:22: error: 'long long int f(int)' redeclared as different kind of symbol
 long long int f(int n)
                      ^
0_0_24962000_12097.cpp:3:15: note: previous declaration 'long long int f'
 long long int f(n);
               ^
0_0_24962000_12097.cpp:24:23: error: 'f' cannot be used as a function
         return 3*f(n-1);
                       ^