subroutine c2f( n, s, ret )
    integer,     intent( in )  :: n
    integer * 1, intent( in )  :: s( n )
    integer,     intent( out ) :: ret
    
    integer :: i, acc, mx, tmp
    
    acc = 0
    mx = -2
    do i = 1, n - 1
        acc = acc + ( s( i ) - iachar( '0', 1 ))
        tmp = i - 1 - 2 * acc
        mx = max( mx, tmp )
    end do
    
    ret = acc + mx + ( s( n ) - iachar( '0', 1 )) + 1
end subroutine c2f

subroutine f( n, s, ret )
    integer,     intent( in )  :: n
    integer * 1, intent( in )  :: s( n )
    integer,     intent( out ) :: ret
    
    integer, parameter :: A97 = 2 * iachar( '0', 1 ) + 1    ! 97
    
    integer     :: i
    integer * 1 :: a( n )           ! temporary pad
    integer     :: cum( n )         ! cumulative sums
    
    ret = count( s == iachar( '1', 1 ))     ! tr/1//
    a = 2 * s - A97                         ! tr/01/\xff\1/r
    a( n ) = 0                              ! "chop", kind of
    
    cum( 1 ) = a( 1 )
    do i = 2, n
        cum( i ) = cum( i - 1 ) + a( i )
    end do
    
    ret = ret - minval( cum )
end subroutine f