implicit none
integer(8) A,B,f,i,k,m,n
integer(8),allocatable :: x(:)
read(*,*) k,n,f
allocate (x(f))
read(*,*) (x(m),m=1,f)
A=0
do m=1,f
   A=A+x(m)
end do
B=k*n-A
if (B>0) then
    write(*,*) B
else
    write(*,*) "-1"
end if
end