program ABC148E
    integer(16) N
    integer(16) ans
    integer(16) i,div
    read*,N
    if(mod(N,2)==1)then
        print"(i0)",0
        stop
    endif
    ans=0;div=10
    do i=1,30
        ans=ans+N/div
        div=div*5
    end do
    print"(i0)",ans
    contains
end program