#include<stdio.h>
long  x;
int n,t,s,p[100001],c[100001],i,j;
int main()
{freopen("branza.in","r",stdin);
 freopen("branza.out","w",stdout);
 scanf("%d %d %d",&n,&s,&t);
 for(i=1;i<=n;i++)
  scanf("%d %d",&p[i],&c[i]);
 for(i=1;i<n;i++)
  for(j=1;j<=t;j++)
   if(p[i]+s*j<p[i+j])
	   p[i+j]=p[i]+s*j;
 for(i=1;i<=n;i++)
  x+=p[i]*c[i];
 printf("%d",x);
 fclose(stdout);
 return 0;}