user: Please show me the C implementation that matches the assembly code listed below:

```asm
<trees>:
  endbr64
  push   %r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  sub    $0x78,%rsp
  mov    %edi,0x6c(%rsp)
  movq   $0x1,0x60(%rsp)
  cmp    $0x1,%edi
  jbe    L648
  mov    %edi,%eax
  movl   $0x0,0x5c(%rsp)
  movq   $0x0,0x60(%rsp)
  sub    $0x1,%eax
  mov    %eax,0x68(%rsp)
  movq   $0x1,0x48(%rsp)
L289:
  mov    0x68(%rsp),%eax
  cmp    $0x1,%eax
  jbe    L19
  sub    $0x2,%eax
  movq   $0x0,0x50(%rsp)
  mov    %eax,0x44(%rsp)
  movl   $0x0,0x40(%rsp)
  movq   $0x1,0x38(%rsp)
L826:
  mov    0x44(%rsp),%edx
  lea    0x1(%rdx),%eax
  cmp    $0x1,%eax
  jbe    L691
  mov    %edx,0x18(%rsp)
  movq   $0x0,0x20(%rsp)
  movl   $0x0,0x10(%rsp)
  movq   $0x1,(%rsp)
L528:
  mov    0x18(%rsp),%eax
  cmp    $0x1,%eax
  jbe    L730
  sub    $0x1,%eax
  xor    %r12d,%r12d
  mov    $0x1,%ebp
  movq   $0x0,0x8(%rsp)
  mov    %eax,0x14(%rsp)
L498:
  mov    0x14(%rsp),%eax
  cmp    $0x1,%eax
  jbe    L459
  lea    -0x1(%rax),%r15d
  xor    %r14d,%r14d
  mov    %r15d,%r13d
L343:
  mov    %r15d,%edi
  sub    %r13d,%edi
  call   <trees>
  mov    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  lea    (%r14,%rbx,2),%r14
  sub    $0x1,%r13d
  jae    L343
  imul   %r14,%rbp
L459:
  mov    0x8(%rsp),%rax
  lea    0x1(%r12),%r15d
  lea    (%rax,%rbp,2),%rax
  mov    %rax,0x8(%rsp)
  cmp    0x18(%rsp),%r15d
  je     L472
  cmp    $0x1,%r15d
  jbe    L563
  xor    %ebp,%ebp
  xor    %r13d,%r13d
L692:
  mov    %r13d,%edi
  call   <trees>
  mov    %r12d,%edi
  sub    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  mov    %r13d,%eax
  add    $0x1,%r13d
  lea    0x0(%rbp,%rbx,2),%rbp
  cmp    %r12d,%eax
  jne    L692
L907:
  subl   $0x1,0x14(%rsp)
  mov    %r15d,%r12d
  jmp    L498
  nopw   0x0(%rax,%rax,1)
L472:
  mov    (%rsp),%rax
  imul   0x8(%rsp),%rax
  mov    %rax,(%rsp)
L730:
  mov    0x20(%rsp),%rax
  mov    (%rsp),%rsi
  lea    (%rax,%rsi,2),%rax
  mov    %rax,0x20(%rsp)
  mov    0x10(%rsp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,0x14(%rsp)
  cmp    %eax,0x44(%rsp)
  je     L25
  cmpl   $0x1,0x14(%rsp)
  jbe    L658
  mov    %eax,0x8(%rsp)
  xor    %r12d,%r12d
  mov    $0x1,%ebp
  movq   $0x0,(%rsp)
L453:
  mov    0x8(%rsp),%eax
  cmp    $0x1,%eax
  jbe    L763
  lea    -0x1(%rax),%r15d
  xor    %r14d,%r14d
  mov    %r15d,%r13d
L425:
  mov    %r15d,%edi
  sub    %r13d,%edi
  call   <trees>
  mov    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  lea    (%r14,%rbx,2),%r14
  sub    $0x1,%r13d
  jae    L425
  imul   %r14,%rbp
L763:
  mov    (%rsp),%rax
  lea    0x1(%r12),%r15d
  lea    (%rax,%rbp,2),%rax
  mov    %rax,(%rsp)
  cmp    %r12d,0x10(%rsp)
  je     L267
  cmp    $0x1,%r15d
  jbe    L300
  xor    %ebp,%ebp
  xor    %r13d,%r13d
L251:
  mov    %r13d,%edi
  call   <trees>
  mov    %r12d,%edi
  sub    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  mov    %r13d,%eax
  add    $0x1,%r13d
  lea    0x0(%rbp,%rbx,2),%rbp
  cmp    %r12d,%eax
  jne    L251
  subl   $0x1,0x8(%rsp)
  mov    %r15d,%r12d
  jmp    L453
L658:
  movq   $0x1,(%rsp)
  cs nopw 0x0(%rax,%rax,1)
L267:
  mov    0x14(%rsp),%eax
  subl   $0x1,0x18(%rsp)
  mov    %eax,0x10(%rsp)
  jmp    L528
  nopw   0x0(%rax,%rax,1)
L25:
  mov    0x38(%rsp),%rax
  imul   0x20(%rsp),%rax
  mov    %rax,0x38(%rsp)
L691:
  mov    0x50(%rsp),%rax
  mov    0x38(%rsp),%rsi
  lea    (%rax,%rsi,2),%rax
  mov    %rax,0x50(%rsp)
  mov    0x40(%rsp),%eax
  add    $0x1,%eax
  mov    %eax,0x58(%rsp)
  cmp    0x68(%rsp),%eax
  je     L223
  cmpl   $0x1,0x58(%rsp)
  jbe    L839
  mov    0x40(%rsp),%eax
  movl   $0x0,0x30(%rsp)
  movq   $0x0,0x38(%rsp)
  mov    %eax,0x34(%rsp)
  movq   $0x1,0x18(%rsp)
L340:
  mov    0x34(%rsp),%eax
  cmp    $0x1,%eax
  jbe    L130
  sub    $0x1,%eax
  movl   $0x0,0x10(%rsp)
  mov    %eax,0x14(%rsp)
  movq   $0x0,0x20(%rsp)
  movq   $0x1,(%rsp)
L534:
  mov    0x14(%rsp),%eax
  cmp    $0x1,%eax
  jbe    L1
  lea    -0x1(%rax),%r15d
  xor    %r14d,%r14d
  xor    %r12d,%r12d
  mov    $0x1,%ebp
  mov    %r15d,0x8(%rsp)
L592:
  mov    0x8(%rsp),%edi
  lea    0x1(%r12),%r13d
  sub    %r12d,%edi
  call   <trees>
  imul   %rbp,%rax
  lea    (%r14,%rax,2),%r14
  cmp    0x14(%rsp),%r13d
  je     L429
  cmp    $0x1,%r13d
  jbe    L476
  xor    %ebp,%ebp
  xor    %r15d,%r15d
L411:
  mov    %r15d,%edi
  call   <trees>
  mov    %r12d,%edi
  sub    %r15d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  mov    %r15d,%eax
  add    $0x1,%r15d
  lea    0x0(%rbp,%rbx,2),%rbp
  cmp    %r12d,%eax
  jne    L411
L197:
  mov    %r13d,%r12d
  jmp    L592
  nopw   0x0(%rax,%rax,1)
L429:
  mov    (%rsp),%rax
  imul   %r14,%rax
  mov    %rax,(%rsp)
L1:
  mov    0x20(%rsp),%rax
  mov    (%rsp),%rdx
  lea    (%rax,%rdx,2),%rax
  mov    %rax,0x20(%rsp)
  mov    0x10(%rsp),%eax
  add    $0x1,%eax
  mov    %eax,0x28(%rsp)
  cmp    0x34(%rsp),%eax
  je     L234
  cmpl   $0x1,0x28(%rsp)
  jbe    L236
  mov    0x10(%rsp),%eax
  movq   $0x0,(%rsp)
  xor    %r12d,%r12d
  mov    $0x1,%ebp
  mov    %eax,0x8(%rsp)
L239:
  mov    0x8(%rsp),%eax
  cmp    $0x1,%eax
  jbe    L547
  lea    -0x1(%rax),%r15d
  xor    %r14d,%r14d
  mov    %r15d,%r13d
L506:
  mov    %r15d,%edi
  sub    %r13d,%edi
  call   <trees>
  mov    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  lea    (%r14,%rbx,2),%r14
  sub    $0x1,%r13d
  jae    L506
  imul   %r14,%rbp
L547:
  mov    (%rsp),%rax
  lea    0x1(%r12),%r15d
  lea    (%rax,%rbp,2),%rax
  mov    %rax,(%rsp)
  cmp    %r12d,0x10(%rsp)
  je     L855
  cmp    $0x1,%r15d
  jbe    L11
  xor    %ebp,%ebp
  xor    %r13d,%r13d
L908:
  mov    %r13d,%edi
  call   <trees>
  mov    %r12d,%edi
  sub    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  mov    %r13d,%eax
  add    $0x1,%r13d
  lea    0x0(%rbp,%rbx,2),%rbp
  cmp    %r12d,%eax
  jne    L908
  subl   $0x1,0x8(%rsp)
  mov    %r15d,%r12d
  jmp    L239
L236:
  movq   $0x1,(%rsp)
  nopl   0x0(%rax,%rax,1)
L855:
  mov    0x28(%rsp),%eax
  subl   $0x1,0x14(%rsp)
  mov    %eax,0x10(%rsp)
  jmp    L534
  nopw   0x0(%rax,%rax,1)
L234:
  mov    0x18(%rsp),%rax
  imul   0x20(%rsp),%rax
  mov    %rax,0x18(%rsp)
L130:
  mov    0x38(%rsp),%rax
  mov    0x18(%rsp),%rsi
  lea    (%rax,%rsi,2),%rax
  mov    %rax,0x38(%rsp)
  mov    0x30(%rsp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,0x8(%rsp)
  cmp    %eax,0x40(%rsp)
  je     L148
  cmpl   $0x1,0x8(%rsp)
  jbe    L180
  mov    %eax,(%rsp)
  xor    %r12d,%r12d
  mov    $0x1,%ebp
  movq   $0x0,0x18(%rsp)
L207:
  mov    (%rsp),%eax
  cmp    $0x1,%eax
  jbe    L122
  lea    -0x1(%rax),%r15d
  xor    %r14d,%r14d
  mov    %r15d,%r13d
L728:
  mov    %r15d,%edi
  sub    %r13d,%edi
  call   <trees>
  mov    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  lea    (%r14,%rbx,2),%r14
  sub    $0x1,%r13d
  jae    L728
  imul   %r14,%rbp
L122:
  mov    0x18(%rsp),%rax
  lea    0x1(%r12),%r15d
  lea    (%rax,%rbp,2),%rax
  mov    %rax,0x18(%rsp)
  cmp    0x30(%rsp),%r12d
  je     L729
  cmp    $0x1,%r15d
  jbe    L801
  xor    %ebp,%ebp
  xor    %r13d,%r13d
L247:
  mov    %r13d,%edi
  call   <trees>
  mov    %r12d,%edi
  sub    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  mov    %r13d,%eax
  add    $0x1,%r13d
  lea    0x0(%rbp,%rbx,2),%rbp
  cmp    %r12d,%eax
  jne    L247
  subl   $0x1,(%rsp)
  mov    %r15d,%r12d
  jmp    L207
L180:
  movq   $0x1,0x18(%rsp)
  nopw   0x0(%rax,%rax,1)
L729:
  mov    0x8(%rsp),%eax
  subl   $0x1,0x34(%rsp)
  mov    %eax,0x30(%rsp)
  jmp    L340
  nopw   0x0(%rax,%rax,1)
L11:
  subl   $0x1,0x8(%rsp)
  mov    $0x1,%ebp
  mov    %r15d,%r12d
  jmp    L239
L839:
  movq   $0x1,0x38(%rsp)
L148:
  mov    0x58(%rsp),%eax
  subl   $0x1,0x44(%rsp)
  mov    %eax,0x40(%rsp)
  jmp    L826
L223:
  mov    0x48(%rsp),%rax
  imul   0x50(%rsp),%rax
  mov    %rax,0x48(%rsp)
L19:
  mov    0x60(%rsp),%rax
  mov    0x48(%rsp),%rsi
  lea    (%rax,%rsi,2),%rax
  mov    %rax,0x60(%rsp)
  mov    0x5c(%rsp),%eax
  add    $0x1,%eax
  mov    %eax,0x40(%rsp)
  cmp    %eax,0x6c(%rsp)
  je     L648
  cmpl   $0x1,0x40(%rsp)
  jbe    L480
  mov    0x5c(%rsp),%eax
  movl   $0x0,0x30(%rsp)
  movq   $0x0,0x48(%rsp)
  lea    -0x1(%rax),%esi
  mov    %eax,0x34(%rsp)
  mov    %esi,0x38(%rsp)
  movq   $0x1,0x20(%rsp)
L205:
  cmpl   $0x1,0x34(%rsp)
  jbe    L249
  mov    0x38(%rsp),%eax
  movl   $0x0,0x10(%rsp)
  movq   $0x0,0x28(%rsp)
  mov    %eax,0x18(%rsp)
  movq   $0x1,(%rsp)
L876:
  mov    0x18(%rsp),%eax
  cmp    $0x1,%eax
  jbe    L382
  sub    $0x1,%eax
  xor    %r12d,%r12d
  mov    $0x1,%ebp
  movq   $0x0,0x8(%rsp)
  mov    %eax,0x14(%rsp)
L722:
  mov    0x14(%rsp),%eax
  cmp    $0x1,%eax
  jbe    L678
  lea    -0x1(%rax),%r15d
  xor    %r14d,%r14d
  mov    %r15d,%r13d
L128:
  mov    %r15d,%edi
  sub    %r13d,%edi
  call   <trees>
  mov    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  lea    (%r14,%rbx,2),%r14
  sub    $0x1,%r13d
  jae    L128
  imul   %r14,%rbp
L678:
  mov    0x8(%rsp),%rax
  lea    0x1(%r12),%r15d
  lea    (%rax,%rbp,2),%rax
  mov    %rax,0x8(%rsp)
  cmp    0x18(%rsp),%r15d
  je     L424
  cmp    $0x1,%r15d
  jbe    L277
  xor    %ebp,%ebp
  xor    %r13d,%r13d
L79:
  mov    %r13d,%edi
  call   <trees>
  mov    %r12d,%edi
  sub    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  mov    %r13d,%eax
  add    $0x1,%r13d
  lea    0x0(%rbp,%rbx,2),%rbp
  cmp    %r12d,%eax
  jne    L79
L787:
  subl   $0x1,0x14(%rsp)
  mov    %r15d,%r12d
  jmp    L722
  cs nopw 0x0(%rax,%rax,1)
L424:
  mov    (%rsp),%rax
  imul   0x8(%rsp),%rax
  mov    %rax,(%rsp)
L382:
  mov    0x28(%rsp),%rax
  mov    (%rsp),%rdx
  lea    (%rax,%rdx,2),%rax
  mov    %rax,0x28(%rsp)
  mov    0x10(%rsp),%eax
  add    $0x1,%eax
  mov    %eax,0x14(%rsp)
  cmp    0x34(%rsp),%eax
  je     L428
  cmpl   $0x1,0x14(%rsp)
  jbe    L794
  mov    0x10(%rsp),%eax
  movq   $0x0,(%rsp)
  xor    %r12d,%r12d
  mov    $0x1,%ebp
  mov    %eax,0x8(%rsp)
L294:
  mov    0x8(%rsp),%eax
  cmp    $0x1,%eax
  jbe    L184
  lea    -0x1(%rax),%r15d
  xor    %r14d,%r14d
  mov    %r15d,%r13d
L487:
  mov    %r15d,%edi
  sub    %r13d,%edi
  call   <trees>
  mov    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  lea    (%r14,%rbx,2),%r14
  sub    $0x1,%r13d
  jae    L487
  imul   %r14,%rbp
L184:
  mov    (%rsp),%rax
  lea    0x1(%r12),%r15d
  lea    (%rax,%rbp,2),%rax
  mov    %rax,(%rsp)
  cmp    %r12d,0x10(%rsp)
  je     L161
  cmp    $0x1,%r15d
  jbe    L141
  xor    %ebp,%ebp
  xor    %r13d,%r13d
L46:
  mov    %r13d,%edi
  call   <trees>
  mov    %r12d,%edi
  sub    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  mov    %r13d,%eax
  add    $0x1,%r13d
  lea    0x0(%rbp,%rbx,2),%rbp
  cmp    %r12d,%eax
  jne    L46
  subl   $0x1,0x8(%rsp)
  mov    %r15d,%r12d
  jmp    L294
L794:
  movq   $0x1,(%rsp)
  nopw   0x0(%rax,%rax,1)
L161:
  mov    0x14(%rsp),%eax
  subl   $0x1,0x18(%rsp)
  mov    %eax,0x10(%rsp)
  jmp    L876
  nopw   0x0(%rax,%rax,1)
L428:
  mov    0x20(%rsp),%rax
  imul   0x28(%rsp),%rax
  mov    %rax,0x20(%rsp)
L249:
  mov    0x48(%rsp),%rax
  mov    0x20(%rsp),%rcx
  lea    (%rax,%rcx,2),%rax
  mov    %rax,0x48(%rsp)
  mov    0x30(%rsp),%eax
  lea    0x1(%rax),%esi
  mov    %esi,0x28(%rsp)
  cmp    %eax,0x5c(%rsp)
  je     L173
  cmpl   $0x1,0x28(%rsp)
  jbe    L754
  mov    %eax,0x18(%rsp)
  movq   $0x0,0x20(%rsp)
  movl   $0x0,0x10(%rsp)
  movq   $0x1,(%rsp)
L538:
  mov    0x18(%rsp),%eax
  cmp    $0x1,%eax
  jbe    L723
  sub    $0x1,%eax
  xor    %r12d,%r12d
  mov    $0x1,%ebp
  movq   $0x0,0x8(%rsp)
  mov    %eax,0x14(%rsp)
L800:
  mov    0x14(%rsp),%eax
  cmp    $0x1,%eax
  jbe    L229
  lea    -0x1(%rax),%r15d
  xor    %r14d,%r14d
  mov    %r15d,%r13d
L886:
  mov    %r15d,%edi
  sub    %r13d,%edi
  call   <trees>
  mov    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  lea    (%r14,%rbx,2),%r14
  sub    $0x1,%r13d
  jae    L886
  imul   %r14,%rbp
L229:
  mov    0x8(%rsp),%rax
  lea    0x1(%r12),%r15d
  lea    (%rax,%rbp,2),%rax
  mov    %rax,0x8(%rsp)
  cmp    0x18(%rsp),%r15d
  je     L33
  cmp    $0x1,%r15d
  jbe    L546
  xor    %ebp,%ebp
  xor    %r13d,%r13d
L100:
  mov    %r13d,%edi
  call   <trees>
  mov    %r12d,%edi
  sub    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  mov    %r13d,%eax
  add    $0x1,%r13d
  lea    0x0(%rbp,%rbx,2),%rbp
  cmp    %r12d,%eax
  jne    L100
L245:
  subl   $0x1,0x14(%rsp)
  mov    %r15d,%r12d
  jmp    L800
  nopl   0x0(%rax)
L33:
  mov    (%rsp),%rax
  imul   0x8(%rsp),%rax
  mov    %rax,(%rsp)
L723:
  mov    0x20(%rsp),%rax
  mov    (%rsp),%rsi
  lea    (%rax,%rsi,2),%rax
  mov    %rax,0x20(%rsp)
  mov    0x10(%rsp),%eax
  lea    0x1(%rax),%esi
  mov    %esi,0x14(%rsp)
  cmp    0x30(%rsp),%eax
  je     L272
  cmpl   $0x1,0x14(%rsp)
  jbe    L570
  mov    %eax,0x8(%rsp)
  xor    %r12d,%r12d
  mov    $0x1,%ebp
  movq   $0x0,(%rsp)
L347:
  mov    0x8(%rsp),%eax
  cmp    $0x1,%eax
  jbe    L700
  lea    -0x1(%rax),%r15d
  xor    %r14d,%r14d
  mov    %r15d,%r13d
L4:
  mov    %r15d,%edi
  sub    %r13d,%edi
  call   <trees>
  mov    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  lea    (%r14,%rbx,2),%r14
  sub    $0x1,%r13d
  jae    L4
  imul   %r14,%rbp
L700:
  mov    (%rsp),%rax
  lea    0x1(%r12),%r15d
  lea    (%rax,%rbp,2),%rax
  mov    %rax,(%rsp)
  cmp    %r12d,0x10(%rsp)
  je     L544
  cmp    $0x1,%r15d
  jbe    L13
  xor    %ebp,%ebp
  xor    %r13d,%r13d
L369:
  mov    %r13d,%edi
  call   <trees>
  mov    %r12d,%edi
  sub    %r13d,%edi
  mov    %rax,%rbx
  call   <trees>
  imul   %rax,%rbx
  mov    %r13d,%eax
  add    $0x1,%r13d
  lea    0x0(%rbp,%rbx,2),%rbp
  cmp    %r12d,%eax
  jne    L369
  subl   $0x1,0x8(%rsp)
  mov    %r15d,%r12d
  jmp    L347
L570:
  movq   $0x1,(%rsp)
  cs nopw 0x0(%rax,%rax,1)
L544:
  mov    0x14(%rsp),%eax
  subl   $0x1,0x18(%rsp)
  mov    %eax,0x10(%rsp)
  jmp    L538
L754:
  movq   $0x1,0x20(%rsp)
  nopl   0x0(%rax,%rax,1)
L272:
  mov    0x28(%rsp),%eax
  subl   $0x1,0x34(%rsp)
  subl   $0x1,0x38(%rsp)
  mov    %eax,0x30(%rsp)
  jmp    L205
L141:
  subl   $0x1,0x8(%rsp)
  mov    $0x1,%ebp
  mov    %r15d,%r12d
  jmp    L294
L13:
  subl   $0x1,0x8(%rsp)
  mov    $0x1,%ebp
  mov    %r15d,%r12d
  jmp    L347
L801:
  subl   $0x1,(%rsp)
  mov    $0x1,%ebp
  mov    %r15d,%r12d
  jmp    L207
L300:
  subl   $0x1,0x8(%rsp)
  mov    $0x1,%ebp
  mov    %r15d,%r12d
  jmp    L453
L480:
  movq   $0x1,0x48(%rsp)
L173:
  mov    0x40(%rsp),%eax
  subl   $0x1,0x68(%rsp)
  mov    %eax,0x5c(%rsp)
  jmp    L289
L648:
  mov    0x60(%rsp),%rax
  add    $0x78,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
L277:
  mov    $0x1,%ebp
  jmp    L787
L476:
  mov    $0x1,%ebp
  jmp    L197
L563:
  mov    $0x1,%ebp
  jmp    L907
L546:
  mov    $0x1,%ebp
  jmp    L245
```

assistant: ```c
unsigned long long trees(unsigned n) {
  if (n <= 1) {
    return 1;
  }
  unsigned long long res = 0;
  unsigned i;
  for (i = 0; i < n; i++) {
    res += 2 * trees(i) * trees(n - i - 1);
  }
  return res;
}
```