前缀和+推公式
#include <iostream>
#include <cstring>
using namespace std;
typedef long long LL;
const int N = 200010;
int n, a[N], sum[N];
LL res;
int main()
{
cin >> n;
for (int i = 1; i <= n; i ++ ) {
cin >> a[i];
sum[i] = sum[i - 1] + a[i]; // 处理前缀和
}
for (int i = 1; i <= n - 1; i ++ ) {
res += (LL)a[i] * (sum[n] - sum[i]); // 推出来的公式
}
cout << res;
return 0;
}