问题描述：
解题思路：
#include <iostream>
using namespace std;
int shumu, yushuan, l, r, ans;
int set = 1;
int main()
{
cin >> shumu >> yushuan;
cin >> l;
while (cin >> r)
{
ans += (r - l) * set;
l = r;
set++;
}
cout << (ans + set * (yushuan - r));
}