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

typedef long long ll;
typedef double dl;

#define endl "\n"
#define vi vector<int>
#define mii map<int,int>
#define mci map<char,int>
#define optimize() ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define fraction() cout.unsetf(ios::floatfield); cout.precision(10); cout.setf(ios::fixed,ios::floatfield);
#define yes cout<<"YES"<<endl;
#define no cout<<"NO"<<endl;

void solve(){
    ll n1;
    cin >> n1 ;

    char l;  cin >> l ;

    ll  n2 ;
    cin >> n2;

    string llk ;
    cin >> llk ;

    ll n ;  cin >> n;

    ll d = n2-n1;

    ll x = d-n1;

    ll nn = n*d - n1;

    ll ans = (n1 + nn) * n;
    if(ans % 2){
        cout << (ans/2)+1 << endl;
    }
    else{
        cout << ans / 2 << endl;
    }

}

int main()
{
    optimize();

    int t=1;
    //cin >> t;

    while(t--)  solve();

    return 0;
}