#include<stdio.h>
int dq[1<<17],st=1,dr=0,s=0,t;
int cost[1<<17],kg[1<<17];
inline void stanga (int i)
{
	if(i-dq[st]==t)
		++st;
}
void dreapta(int i)
{
	while(st<=dr && cost[i]<=cost[dq[dr]]+s*(i-dq[dr]))
	
		--dr;
	dq[++dr]=i;
	
}
int main()
{
	int n,s,t;
	
	freopen("branza.in","r",stdin);
	freopen("branza.out","w",stdout);
	scanf("%d%d%d",&n,&s,&t);
	for(int i=1;i<=n;++i)
		scanf("%d%d",&cost[i],&kg[i]);
	for(int i=1;i<t;++i)
		dreapta(i);
	for(int i=t;i<=n;++i)
	{
		dreapta(i);
		stanga(i);
		s=s+(cost[dq[st]]+s*(i-dq[st]))*kg[i];
	}
	printf("%d",s);
	return 0;
}