Problem:
Score : 300 points

Problem Statement

Takahashi, Nakahashi and Hikuhashi have integers A, B and C, respectively.
After repeating the following operation K times, find the integer Takahashi will get minus the integer Nakahashi will get:

Each of them simultaneously calculate the sum of the integers that the other two people have, then replace his own integer with the result.

However, if the absolute value of the answer exceeds 10^{18}, print Unfair instead.

Constraints

1 \leq A,B,C \leq 10^9

0 \leq K \leq 10^{18}

All values in input are integers.

Input

Input is given from Standard Input in the following format:

A B C K

Output

Print the integer Takahashi will get minus the integer Nakahashi will get, after repeating the following operation K times.
If the absolute value of the answer exceeds 10^{18}, print Unfair instead.

Sample Input 1

1 2 3 1

Sample Output 1

1

After one operation, Takahashi, Nakahashi and Hikuhashi have 5, 4 and 3, respectively. We should print 5-4=1.

Sample Input 2

2 3 2 0

Sample Output 2

-1

Sample Input 3

1000000000 1000000000 1000000000 1000000000000000000

Sample Output 3

0

Solution in C++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<bits/stdc++.h>
using namespace std;

#define SPEED ios::sync_with_stdio(false); cin.tie(0); cout.tie(0)
#define read() 	        freopen("in.txt","r",stdin)
#define write()         freopen("out.txt","w",stdout)
#define sf(n) 	        scanf("%d",&n)
#define sfd(n)          scanf("%lf",&n)
#define sl(x)           scanf("%I64d",&x)
#define sfl(n) 	        scanf("%lld", &n)
#define sfc(n)          scanf(" %c",&n)
#define sful(n)         scanf("%ull",&n)
#define ull             unsigned long long int
#define endll           pf("\n")
#define pf              printf
#define pi              acos(-1.0)
#define eps 	        1e-10
#define mem(a,b)        memset(a,b,sizeof(a))
#define pb              push_back
#define xx              first
#define yy              second
#define pii             pair<int, int>
#define pll             pair<ll, ll>
#define MP make_pair
#define ll              long long
#define uniq(a)      a.erase(unique(a.begin(), a.end()), a.end())
#define cnd tree[idx]
#define lnd tree[idx<<1]
#define rnd tree[(idx<<1)+1]
#define lndp (idx<<1),(b),((b+e)>>1)
#define rndp ((idx<<1)+1),(((b+e)>>1)+1),(e)
#define inf 1999999999
#define MX 100005
#define mod 1000000007
#define var    int i,j,k,n,m,z=0,c=0,t,x,y,l,r,mid
//int dx[8]={0,0,1,1,-1,-1,1,-1};  //8 direction
//int dy[8]={1,-1,1,-1,1,-1,0,0};
//int dx[4]={1,-1,0,0};  //4 direction
//int dy[4]={0,0,1,-1};

int main()
{
    ll var;
    sfl(n);sfl(m);sfl(k);sfl(x);
    z=abs(n-m);
    if(k%2==0)
    {
        if(n>=m && z<=1000000000000000000LL)
        {
            cout<<z<<endl;
        }
        else if(n<m && z<=1000000000000000000LL)
        {
            cout<<-z<<endl;
        }
        else cout<<"Unfair"<<endl;
    }
    else
    {
        if(n>=m && z<=1000000000000000000LL)
        {
            cout<<-z<<endl;
        }
        else if(n<m && z<=1000000000000000000LL)
        {
            cout<<z<<endl;
        }
        else cout<<"Unfair"<<endl;
    }
}