P4781 拉格朗日插值

#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,n) for (int i=a;i<n;i++)
#define per(i,a,n) for (int i=n-1;i>=a;i--)
#define pb push_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define SZ(x) ((int)(x).size())
typedef long long ll;
typedef pair<int,int> Pii;
const ll mod=;
const int maxn = 2e3+;
ll powmod(ll a,ll b) {ll res=;a%=mod; assert(b>=); for(;b;b>>=){if(b&)res=res*a%mod;a=a*a%mod;}return res;}
// head
int x[maxn],y[maxn];
int main(){

    int n,k;
    scanf("%d%d",&n,&k);
    for(int i=;i<=n;i++) scanf("%d%d",&x[i],&y[i]);
    ll sum=;
    for(int i=;i<=n;i++){

        ll fz=y[i],fm=;
        for(int j=;j<=n;j++){

            if(j==i) continue;
            fz=(fz*(k-x[j])%mod+mod)%mod;
            fm=(fm*(x[i]-x[j])%mod+mod)%mod;
        }
        fm=powmod(fm,mod-);
        sum=(sum+fz*fm%mod)%mod;
    }
    printf("%lld\n",sum);
}