CCF-CSP 202112-1 序列查询（C++）

#include<iostream>
using namespace std;

int main() {
	std::ios::sync_with_stdio(false);

	int n, N;
	cin >> n >> N;
	int sum = 0, preNum = 0;
	for (int i = 1; i <= n; ++i) {
		int num;
		cin >> num;
		sum += (num - preNum) * (i - 1);
		preNum = num;
		if (i == n) {
			sum += (N - num) * i;
		}
	}
	cout << sum << endl;
	return 0;
}