There is my solution:

Program t1114;

Const MaxL=15;

Var Mas         :array[1..20,0..MaxL,0..MaxL]of comp;
    Ans         :comp;
    N,i,j,A,B,k :longint;
    u,v         :longint;

begin
Read(N,A,B);
for i:=1 to 20 do
 for j:=1 to MaxL do
  for k:=1 to MaxL do
   mas[i,j,k]:=0;
if A>0 then mas[1,A-1,B]:=1;
if B>0 then mas[1,A,B-1]:=1;
if (A>0)and(B>0) then mas[1,A-1,B-1]:=1;
mas[1,A,B]:=1;
for i:=2 to N do begin
 for j:=0 to MaxL do
  for k:=0 to MaxL do
   if mas[i-1,j,k]<>0 then begin
    for u:=0 to j do
     for v:=0 to k do
      mas[i,j-u,k-v]:=mas[i,j-u,k-v]+mas[i-1,j,k];
   end;
end;
Ans:=0;
for i:=0 to MaxL do
 for j:=0 to MaxL do
  Ans:=Ans+mas[N,i,j];
writeln(Ans:0:0);
end.

There is my AC solution:

Program t1114;

Const MaxL=15;

Var Mas         :array[1..20,0..MaxL,0..MaxL]of extended;
    Ans         :extended;
    N,i,j,A,B,k :longint;
    u,v         :longint;

begin
Read(N,A,B);
for i:=1 to 20 do
 for j:=1 to MaxL do
  for k:=1 to MaxL do
   mas[i,j,k]:=0;
for u:=0 to A do
 for v:=0 to B do
      mas[1,A-u,B-v]:=1;
for i:=2 to N do
 for j:=0 to MaxL do
  for k:=0 to MaxL do
   if mas[i-1,j,k]<>0 then
    for u:=0 to j do
     for v:=0 to k do
      mas[i,j-u,k-v]:=mas[i,j-u,k-v]+mas[i-1,j,k];
Ans:=0;
for i:=0 to MaxL do
 for j:=0 to MaxL do
  Ans:=Ans+mas[N,i,j];
writeln(Ans:0:0);
end.