#include<bits/stdc++.h>
#define FOR(i,a,b,k) for (ll i=a;i<=b;i+=k)
#define FORD(i,a,b,k) for (ll i=a;i>=b;i-=k)
#define pb(i) push_back(i)
#define ll long long int
#define MOD 1000000007
#define lim 1e16
using namespace std;
ll a,b,c,res=0;
 
int main()
{
    	ios_base::sync_with_stdio(false);
        cin.tie(NULL);cout.tie(NULL);
    cin>>a>>b>>c;
 
    if(c%2==0)
	{
		if(a%2==0) {res++;a++;}
		if(b%2==0) {res++;b--;}
 
		res+=(b-a)/2;
	}
	else 
	{
		ll them_a=c-1-a%c;
		ll chuky=(c+1)/2;
		ll tanso=(b-a)/c;
 
		a+=them_a+tanso*c;
		res+=them_a/2+them_a%2+tanso*chuky;
 
		res+=(b-a)/2+(b-a)%2;
	}
 
	cout<<res;
    return 0;
}

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