pascal鸡和兔子若干,键盘输入鸡和兔子总的头数和腿数,求鸡和兔子各有多少只

egin
read(a,b); {a为总头数,b为腿数}
for x:=1 to a do {x为鸡的头数}
for y:=1 to a do {y为兔子头数}
if x*2+y*4=b then write('hen',x,' ','rabbit',y) ;
end.