implicit none
character*4 head1,head2,ext
character*10 fn1,fn2
integer i,kev
head1='jack'
head2='jill'
ext='.dat'
do i=1, 50000
kev=int((i-1)/50)+1000
if(mod(i,50).eq.0) then
write(fn1,'(a,i4,a)') head1,kev,ext
write(3,*) fn1
write(fn2,'(a,i4,a)') head2,kev,ext...