Main.c: In function 'main':
Main.c:11:50: warning: implicit declaration of function 'factorial' [-Wimplicit-function-declaration]
   11 |         sum += (unsigned long long)m * (m - 1) * factorial(m - 1);
      |                                                  ^~~~~~~~~
Main.c: At top level:
Main.c:25:20: error: conflicting types for 'factorial'; have 'long long unsigned int(int)'
   25 | unsigned long long factorial(int n) {
      |                    ^~~~~~~~~
Main.c:11:50: note: previous implicit declaration of 'factorial' with type 'int()'
   11 |         sum += (unsigned long long)m * (m - 1) * factorial(m - 1);
      |                                                  ^~~~~~~~~