program ex
implicit none
real::x,x1,x2,func
real::dy,delt=1.e-6

read(*,*) x1,x2
if(func(x1)*func(x2) > 0.0) then
write(*,*) 'boo boo'
stop
end if
dy = sqrt(-func(x1)*func(x2))

do while (dy > delt)
x=(x1+x2)/2.0
if(func(x1)*func(x) > 0.0) then
x1=x
　else
x2=x
end if
dy = sqrt(-func(x1)*func(x2))
end do

x=x1
if(abs(func(x1)) > abs(func(x2))) x=x2
write(*,*) x
end