#include <stdio.h>
#define LL long long
LL ans;
int main(){
	int n,i,m,a,b;
	scanf("%d",&n);
	for(i=1;i<=n;i++){
		m=n/i;
		a=i,b=a+(m-1)*i;
		ans+=(LL)(a+b)*m/2;
	}
	printf("%lld\n",ans);
	return 0;
}