【题解】【上海】【差分求和】

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 5e5 + 10;
ll a[N],b[N];
ll sum = 0;
int main(){
	int n;
	cin >> n;
	for(int i = 1; i <= n; i++){
		cin >> a[i];
		b[i] = a[i] + b[i - 1];
	}
	sum = a[2] - a[1];
	for(int i = 3; i <= n; i++){
		sum -= (i - 1) * a[i] - b[i - 1];
	}
	cout << sum << endl;
	return 0;
}