Busil wants to be friends

Interview

Time limit1sMemory limit1024 MB

Summary

Count how many values from A to B differ from K by at most X, and print IMPOSSIBLE if no value qualifies.

Level: Easy1 of 10
Topics: Intervals, Math
Solved: No attempts yet

Problem

Now that all of Busil's friends have become GOSU, Busil wants to make new friends with skill levels close to his own. Everywhere he looks, there are only GOSU, but Busil has managed to gather a few people at his level. Surprisingly, each of these people solved a different number of problems. When they are arranged in order of how many problems they solved, from fewest to most, the counts are $A, A+1, \cdots, B$ .

Some of them differ from Busil by too many problems. Busil decides that only people whose solved count differs from his by at most $X$ qualify as friends. When Busil has solved $K$ problems, find the number of people who can become his friends.

Input

The first line contains $A$ and $B$ , separated by a space. $(1\le A \le B \le 10\,000)$

The second line contains $K$ , the number of problems Busil solved, and a positive integer $X$ , separated by a space. $(1\le K, X \le 10\,000)$

Output

Print the number of people who can be Busil's friends.

If no one can be his friend, print IMPOSSIBLE.

Examples2

Example 1
Input
```
10 100
50 10
```
Expected output
```
21
```
Example 2
Input
```
25 75
10 5
```
Expected output
```
IMPOSSIBLE
```