by hansang:

#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N=1e5+10, M=1e6+10;
LL a[N], c[M]; int n, l, C; 
struct node{LL x, d;} b[N];
void add(LL x, LL w){
    if(x==0) return ;
    for(LL i=x; i>=1; i-=i&-i) c[i]+=w;
}
LL query(LL x){
    LL res=0;
    for(LL i=x; i<=a[n]; i+=i&-i) res+=c[i];
    return res;
}
int main(){
    scanf("%d%d%d", &n, &l, &C);
    for(int i=1; i<=n; i++) scanf("%lld", &a[i]); 
    sort(a+1, a+n+1); LL ans=0;
    for(int i=1; i<=n; i++){
        b[i].x=l*a[i]/a[n];
        b[i].d=l*a[i]%a[n]+1;
        ans+=(i-(n-i+1))*b[i].x;
    }
    memset(c, 0, sizeof(c));
    for(int i=1; i<=n; i++){
        ans-=query(b[i].d+1);
        add(b[i].d, 1);
    }
    printf("%lld\n", ans);
    return 0;
}

by hansang:

#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N=1e5+10, M=1e6+10;
LL a[N], c[M]; int n, l, C; 
struct node{LL x, d;} b[N];
void add(LL x, LL w){
    if(x==0) return ;
    for(LL i=x; i>=1; i-=i&-i) c[i]+=w;
}
LL query(LL x){
    LL res=0;
    for(LL i=x; i<=a[n]; i+=i&-i) res+=c[i];
    return res;
}
int main(){
    scanf("%d%d%d", &n, &l, &C);
    for(int i=1; i<=n; i++) scanf("%lld", &a[i]); 
    sort(a+1, a+n+1); LL ans=0;
    for(int i=1; i<=n; i++){
        b[i].x=l*a[i]/a[n];
        b[i].d=l*a[i]%a[n]+1;
        ans+=(i-(n-i+1))*b[i].x;
    }
    memset(c, 0, sizeof(c));
    for(int i=1; i<=n; i++){
        ans-=query(b[i].d+1);
        add(b[i].d, 1);
    }
    printf("%lld\n", ans);
    return 0;
}