拉格朗日插值 模板

#include<cstdio>
#include<cstring>
using namespace std;
#define N 2010
#define md 998244353
#define ll long long
struct {
	ll x, y;
}a[N];
ll ksm(ll x, ll y) {
	if(!y) return 1;
	ll l = ksm(x, y / 2);
	if(y % 2) return l * l % md * x % md;
	return l * l % md;
}
int main() {
	int n, i, j;
	ll k;
	scanf("%d%lld", &n, &k);
	for(i = 1; i <= n; i++) {
		scanf("%lld%lld", &a[i].x, &a[i].y);
	}
	ll ans = 0;
	for(i = 1; i <= n; i++) {
		ll t = a[i].y, s = 1;
		for(j = 1; j <= n; j++) if(j != i) {
			s = s * (a[i].x - a[j].x) % md;
			t = t * (k - a[j].x) % md;
		}
		ans = (ans + t * ksm(s, md - 2)) % md;
	}
	printf("%lld", (ans % md + md) % md);
	return 0;
}