function N=ShapeFun(s,t)
N1=1/4*(1-s)*(1-t);
N2=1/4*(1+s)*(1-t);
N3=1/4*(1+s)*(1+t);
N4=1/4*(1-s)*(1+t);
N=[N1 0 N2 0 N3 0 N4 0;
0 N1 0 N2 0 N3 0 N4];