#include <stdio.h>
#include<vector>
#include<algorithm>
#include<map>
#include <set>
#include <iostream>

#define ll long long
#define N 10050

using namespace std;

int c[N];
int p[N];

int n, t, cost;
int s;
ll ans;

multiset<int> s1;
vector< multiset<int>::iterator > its;

int main()
{
    freopen("branza.in", "r", stdin);
    freopen("branza.out", "w", stdout);

    scanf("%d", &n);
    scanf("%d", &cost);
    scanf("%d", &t);

    for (int i = 0; i < n/2; i++)
    {
	scanf("%d", &c[i]);
	scanf("%d", &p[i]);
    }

    /*
    for (int i = 0; i < n; i++)
    {
	if (s1.size() > (unsigned int)t )
	{
	    s1.erase(its[i - t - 1]);
	}

	its.push_back(  s1.insert(c[i] - s) );

	ans += (s + (*s1.begin())) * (ll) p[i];
	s += cost;
   }
   */

    printf("%lld", ans);
    return 0;
}