We work from right to left as we would if doing this calculation by hand. In the units column, we have $L-1$ giving 1. Thus, $L=2$. (There is no borrowing required.) In the tens column, we have $3-N$ giving 5. Since 5 is larger than 3, we must borrow from the hundreds column. Thus, $13-N$ gives 5, which means $N=8$. In the hundreds column, we have $(K-1)-4$ giving 4, which means $K=9$. In the thousands column, we have 5 (with nothing borrowed) minus $M$ giving 4. Thus, $5-M=4$ or $M=1$. Finally, $K+L+M+N=9+2+1+8=20$.