Wa on #6,#15,#17,#43

// a~k1*c+1,k2*c+1~b,k1*c+1~k2*c+1
#include<bits/stdc++.h>
using namespace std;
#define int long long
int a,b,c,ans;
signed main(){
    cin>>a>>b>>c;
    int st=(a/c+(a%c!=0))*c;
    ans+=(st+1-a)/2+(st+1-a)%2;//a~st+1
    int fi=(b/c-(b%c==0))*c;
    ans+=(b-fi-1)/2+(b-fi-1)%2;//fi+1~b  
    ans+=(fi-st)/c*(c/2+c%2);//st+1~fi+1 ok
    cout<<ans;
    return 0;
}

//   g++ t558382.cpp -O2 -std=c++14 -Wall -o t558382