Try this

1 1
2 2
3 3
4 4

2 change

There are too many different cases to consider, use a brute-force approach to get AC on the first attempt with one page of code:

1) write a function that checks that the situation is good;

2) for every coordinate make a list of four interesting values (all values taken by existing potatoes and some others if this is not enough);

3) try all possible ways to move none, one, or two potatoes, so that the coordinates of the new position are from the interesting values.

Thenks, I have AC

Edited by author 27.09.2013 15:34

Some test that helped me to go through WA 17, 27, 28, 57

10 1
10 2
10 3
10 4
Changes: 2

1 1
2 1
3 1
2 2
Changes: 1

1 1
1 2
1 3
2 4
Changes: 1

1 1
1 5
10 1
20 1
Changes: 1

I have WA at test #72. Could I get content of this test? Pls :)

Edited by author 07.11.2010 22:46

test#1
1 1
1 2
5 5
5 6
ans#1
1 1
1 2
5 5
5 6
0 change

test#2
1 1
1 2
1 3
1 4
ans#2
2 1
2 2
1 3
1 4
2 change

test#3
1 1
1 2
1 3
2 2
ans#3
1 1
2 4
1 3
2 2
1 change

Edited by author 03.11.2010 18:06

WA53.. Hardly...