莫名其妙 WA 了两个，找不到原因，求助

#include <bits/stdc++.h>
#define int long long

const int N = 1e5 + 10;
const int inf = 2e9;

using namespace std;

inline int read()
{
	int t;
	cin >> t;
	return t;
}

int n, k, a[N], dp[N];

signed main()
{
	ios::sync_with_stdio(false);
	cin.tie(0), cout.tie(0);
	
	n = read(), k = read();
	for(int i = 1;i <= n;i = -~i)	a[i] = read(), dp[i] = inf;	
	
	dp[1] = 0;
	for(int i = 1;i <= n;i = -~i)
	{
		for(int j = max(0LL, i - k); j <= i;j = -~j)
			dp[i] = min(dp[i], dp[j] + abs(a[i] - a[j]));
	}
	
	cout << dp[n];
	return 0;
}