#include<stdio.h>
#define ll long long
long long se,sum;
long n,t,b[101],a[101],j,nr,nr2;
int s;
int main ()
{
    long i;
    freopen("branza.in","r",stdin);
    freopen("branza.out","w",stdout);
    scanf("%ld%d%ld",&n,&s,&t);
    t++;
    for(i=1;i<=n;i++)
        scanf("%ld%ld",&a[i],&b[i]);

    for(i=1;i<=n;i++)
    {
        nr=i-t+1;
        if(nr<1)
            nr=1;
        se=(ll)a[i]*b[i];
        for(j=nr;j<i;j++)
        {
            nr2=a[j]*b[i]+(ll)(i-j)*s*b[i];
            if(i>1 && se>a[j]*b[i]+(ll)(i-j)*s*b[i])
                se=(ll)(i-j)*s*b[i]+a[j]*b[i];
        }
        sum=sum+se;
    }
    printf("%lld",sum);
    return 0;
}