program ex1_13
  implicit none
  real(8) dx,x,y,s,pi
  integer i,n
  real(8) a,b
  write(*,*)'input a:'
  read(*,*)a
  write(*,*)'input b:(a<b)'
  read(*,*)b
  write(*,*)'input n:'
  read(*,*)n
  if(n<1)stop'stop, n<1'
  dx=dble(b-a)/dble(n)
  s=0.0d0
  do i=0,n
     x=dx*dble(i)
     pi=2.0d0*acos(0.0d0)
     y=exp(-x**2/2.0d0)/sqrt(2.0d0*pi)
     if(i==0.or.i==n)then
        s=s+0.5d0*y
     else
        s=s+y
     endif
  enddo
  s=s*dx
  write(*,*)'s=',s
end program ex1_13