#include<cstdio>
int s,sta,dre,n,k,dq[100001],v[100001],cer[100001];
long long nr;
inline void st(int i)
{
    if(i-dq[sta]==k)
        ++sta;
}
void dr(int i)
{
    while(sta<=dre && (long long)v[dq[dre]]+(i-dq[dre])*s>=v[i])
        --dre;
    dq[++dre]=i;
}
int main()
{
    freopen("branza.in","r",stdin);
    freopen("branza.out","w",stdout);
    scanf("%d%d%d",&n,&s,&k);
    for(int i=1;i<=n;++i)
        scanf("%d%d",&v[i],&cer[i]);
    dq[1]=1;
    sta=dre=1;
    nr=v[1]*cer[1];
    for(int i=2;i<=n;i++)
    {
        st(i);
        dr(i);
        nr=nr+(long long)(v[dq[sta]]+(i-dq[sta])*s)*cer[i];
    }
    printf("%lld",nr);
    return 0;
}