program main
  implicit none
  integer(8) a,b,c
  read(*,*)a,b,c
  if(60*a-b<=0)then
    write(*,*)-1
  else
    if(mod(3600*c,(60*a-b))==0)then
      write(*,*)3600*c/(60*a-b)
    else
      write(*,*)3600*c/(60*a-b)+1
    endif
  endif

end program main