const MOD = 998244353n;
const d = require('fs').readFileSync(0, 'utf8').split(/\s+/);
let p = 0;
const n = BigInt(d[p++]), k = BigInt(d[p++]);
const N = Number(n);
const A = new Array(N);
let sumA = 0n, sumB = 0n, sumAB = 0n;
for (let i = 0; i < N; i++) { A[i] = BigInt(d[p++]) % MOD; sumA = (sumA + A[i]) % MOD; }
for (let i = 0; i < N; i++) {
  const b = BigInt(d[p++]) % MOD;
  sumB = (sumB + b) % MOD;
  sumAB = (sumAB + A[i] * b) % MOD;
}
const power = (a, e) => { let r = 1n; a %= MOD; while (e > 0n) { if (e & 1n) r = r * a % MOD; a = a * a % MOD; e >>= 1n; } return r; };
const teamSize = n / k;
const prob = (teamSize - 1n + MOD) % MOD * power((n - 1n) % MOD, MOD - 2n) % MOD;
const total = ((sumA * sumB % MOD) - sumAB % MOD + MOD) % MOD;
console.log((prob * total % MOD).toString());