#include<fstream>
using namespace std;
int n,s,t,a[100001],b[100001],nr,cost,m,i,j,costdz,nror,sum;
int main()
{
    ifstream f("branza.in");
    ofstream g("branza.out");
    f>>n>>s>>t;
    for(i=1;i<=n;i++)
    f>>a[i]>>b[i];
    m=n-t;
    for(i=1;i<=m;i++)
    cost=cost+a[i]*b[i];
    for(i=m+1;i<=n;i++)
    nr=nr+b[i];
    cost=cost+a[m+1]*nr;
        for(i=m+2;i<=n;i++)
        {
            sum=sum+b[i];
        }
        cost=cost+sum*s;
        m++;
        nror=n-m;
        while(nror)
        {
           {
                sum=0;
                for(i=m+2;i<=n;i++)
                sum=sum+b[i];
                cost=cost+sum*s;
                m++;
           }
           nror--;
        }
    g<<cost<<" ";
    return 0;
}