\psset{unit=0.45cm}
%
\pspicture(-14,0)(10,15)
% \showgrid
% The labeled grid
\psset{linewidth=0.01,linecolor=gray}
\multido{\ny=0+1}{15}{
	\psline(-13,\ny)(10,\ny)
}
\multido{\nx=-13+1}{24}{
	\psline(\nx,0)(\nx,14)
}
\multido{\ny=1+1}{13}{
	\rput(-14,\ny){$\scriptscriptstyle\ny$}
}
\setcounter{dummycount}{-4}
\multido{\nx=-12+1}{22}{
	\rput(\nx,-0.5){$\scriptscriptstyle\arabic{dummycount}$}
	\stepcounter{dummycount}
}
% Lines indicating the levels
\psset{linewidth=0.3,linecolor=lightgray,fillstyle=none,linestyle=dashed,dash=0.4 0.6}
%\psline(-13,1)(9,1)
\psline(-13,7)(10,7)
%\psline(-13,13)(9,13)
% Labels
% Points r
\rput(-11.5,1.5){$\scriptstyle r_4$}
\rput(-10.5,1.5){$\scriptstyle r_3$}
\rput(-9.5,1.5){$\scriptstyle r_2$}
\rput(-8.5,1.5){$\scriptstyle r_1$}
% Points s
\rput(-8.5,7.5){$\scriptstyle s_4$}
\rput(-6.5,7.5){$\scriptstyle s_3$}
\rput(-3.5,7.5){$\scriptstyle s_2$}
\rput(-0.5,7.5){$\scriptstyle s_1$}
% Points t
\rput(-0.5,13.5){$\scriptstyle t_4$}
\rput(2.5,13.5){$\scriptstyle t_3$}
\rput(6.5,13.5){$\scriptstyle t_2$}
\rput(9.5,13.5){$\scriptstyle t_1$}
% The lattice paths
\psset{linewidth=0.15,fillstyle=none,linecolor=gray,linestyle=solid}
\psline(-9,1)(-6,1)(-6,3)(-2,3)(-2,5)(-1,5)(-1,7)(-1,8)(3,8)(3,10)(9,10)(9,13)
\psline(-10,1)(-10,2)(-7,2)(-7,4)(-4,4)(-4,7)(-4,9)(1,9)(1,11)(6,11)(6,13)
\psline(-11,1)(-11,5)(-7,5)(-7,7)(-5,7)(-5,11)(0,11)(0,12)(2,12)(2,13)
\psline(-12,1)(-12,7)(-9,7)(-8,7)(-8,9)(-7,9)(-7,11)(-6,11)(-6,13)(-1,13)
% Starting points
\psset{linewidth=0.1,linecolor=black,fillstyle=solid,fillcolor=white}
% Starting points at lower level
\pscircle(-12,1){0.3}
\pscircle(-11,1){0.3}
\pscircle(-10,1){0.3}
\pscircle(-9,1){0.3}
% Starting points at middle level
\pscircle(-9,7){0.3}
\pscircle(-7,7){0.3}
\pscircle(-4,7){0.3}
\pscircle(-1,7){0.3}
% Starting points at upper level
\pscircle(-1,13){0.3}
\pscircle(2,13){0.3}
\pscircle(6,13){0.3}
\pscircle(9,13){0.3}
\endpspicture
