#include<cstdio>
#include<algorithm>
using namespace std;
FILE*in=fopen("branza.in","r");
FILE*out=fopen("branza.out","w");
int n,s,t,c[100005],p,i,j,minn;
long long ras;
int main()
{
    fscanf(in,"%d%d%d",&n,&s,&t);
    for(i=1;i<=n;i++)
    {
        fscanf(in,"%d%d",&c[i],&p);
        minn=c[i];
        int ad=0;
        for(j=i-1;j>=max(1,i-t);j--)
        {
            ad+=s;
            minn=min(minn,ad+c[j]);
        }
        ras+=minn*p;
    }
    fprintf(out,"%lld",ras);
}