piatra

{se da o matrice mare a[m,n], stiind ca este completat cu numere 1,2,...,m*n
diagonala dupa diagonala, serpuit ,
  Dandu-se linia si coloana, sa se calculeze valoarea lui
  k=a[lin,col]}
program serpuit;
var p,min,m,s,n,lin,col,k:longint;
    f,g:text;

begin
  writeln;
  assign(f,'piatra.in');
  reset(f);
  assign(g,'piatra.out');
  rewrite(g);
  readln(f,m,n,lin,col);
  k:=0;
  if m<n then min:=m else min:=n;
  if min+1>lin+col then
    begin
      s:=(lin+col-2)*(lin+col-1) div 2;
      if odd(lin+col) then
        k:=s+lin
      else
        k:=s+col;
    end
  else
    if m+n-min+1<lin+col then
      begin
        p:=m+n-lin-col;
        s:=m*n-p*(p+1) div 2;
        if odd(lin+col) then
            k:=s-m+lin
          else
            k:=s-n+col
      end
    else
      begin
        s:=min*(min+1) div 2;
        p:=col+lin-min-2;
        s:=s+p*min;
        if n<m then
          if odd(lin+col) then
              k:=s+n-col+1
          else
              k:=s+col
        else
          if odd(lin+col) then
            k:=s+lin
          else
            k:=s+m-lin+1;
      end;
  writeln(g,k);
  close(f);
  close(g);
end.