#include <iostream>
#include <vector>
#include <algorithm>
#include <unordered_map>
#include<unordered_set>
#include<string>
#define ll long long
using namespace std;

int main() {
    string s;
    ll n;
    cin>>s;
    cin>>n;
    ll b;
    ll a;
    string s1="", s2="";
    ll index=0;
    for(int i=0; i<s.length(); i++){
        if(isdigit(s[i])){
            s1 += s[i];
        }else {
            index = i;
            break;

        }
    }
    for(int i=index+1; i<s.length(); i++){
        if(isdigit(s[i])){
            s2 += s[i];
        }else break;
    }
    a = stoi(s1);
    b = stoi(s2);
    //cout<<"a="<<a<<" b="<<b<<endl;
    ll d = b-a;
    ll sum = (n*(2*a+(n-1)*d))/2;
    cout<<sum<<endl;
}