а=800000
а-1=799999
а+1=800001
а-1=4999
а=5000
а+1=5001
а+1=20002
а=20001
а-1=20000
a-1 a a+1
799999 800000 800001
4999 5000 5001
20000 20001 20002
а=800000
а-1=799999
а+1=800001
а-1=4999
а=5000
а+1=5001
а+1=20002
а=20001
а-1=20000
a-1 a a+1
799999 800000 800001
4999 5000 5001
20000 20001 20002