by hansang:

#include<bits/stdc++.h>
using namespace std;
const int N=1e5+10;
typedef long long LL;
struct node{LL t, d;} a[N];
bool cmp(node n1, node n2){
    return n1.t*n2.d < n2.t*n1.d;
}
int main(){
    int n; scanf("%d", &n);
    for(int i=1; i<=n; i++){
        scanf("%lld%lld", &a[i].t, &a[i].d);
        a[i].t *= 2;
    }
    sort(a+1, a+n+1, cmp);
    LL sum=0, ans=0;
    for(int i=1; i<=n; i++){
        ans += sum * a[i].d;
        sum += a[i].t;
    }
    printf("%lld\n", ans);
    return 0;
}

by hansang:

#include<bits/stdc++.h>
using namespace std;
const int N=1e5+10;
typedef long long LL;
struct node{LL t, d;} a[N];
bool cmp(node n1, node n2){
	return n1.t*n2.d<n2.t*n1.d;
}
int main(){
	int n; scanf("%d", &n);
	for(int i=1; i<=n; i++){
		scanf("%lld%lld", &a[i].t, &a[i].d);
		a[i].t*=2;
	}
	sort(a+1, a+n+1, cmp);
	LL sum=0, ans=0;
	for(int i=1; i<=n; i++){
		ans+=sum*a[i].d;
		sum+=a[i].t;
	}
	printf("%lld\n", ans);
	return 0;
}