program abc
implicit none
real::x,y,t
write(*,*)'Please input x,y'
read(*,*),x,y
if(x>=0,y>0) then t=x+y
,elseif(x>0,y<0) then t=x-y,
elseif(x<0)then t=y,
elseif(x=0ory=0) then t=0
print*,'t=',t
endprogram abc