#include <iostream>
#include<fstream>
#include<deque>
#include<vector>
using namespace std;
ifstream f("branza.in");
ofstream g("branza.out");
int N,S,T,s=0,nr=0,i,x,y,b,ok=1,a;
vector<int> C;
deque<int> P;
int main()
{
    f>>N>>S>>T;
    for(i=0;i<T-1;i++)
    {
        f>>x>>y;
        s=s+x*y;
    }
    ok=1;
    for(i=T-1;i<N;i++)
    {
        f>>x>>y;
        if(ok==1)
        {
            b=x;
            ok=0;
        }
        P.push_front(y);
    }
    a=P.back();
    P.pop_back();

    while(!P.empty())
    {
        nr=nr+P.front();
        s=s+S*nr;
        P.pop_front();
    }
    a=a+nr;
    s=s+a*b;
    g<<s;
    return 0;
}