#include <stdio.h>

int main () {
  int n = 0;
  int m = 0;
  long long a[200000] = {};
  
  int res = 0;
  
  long long ans = 1LL;
  long long mod_num = 998244353LL;
  
  long long powm1[200000] = {};
  
  long long sum[2] = {};
  int p = 0;
  
  res = scanf("%d", &n);
  res = scanf("%d", &m);
  for (int i = 0; i < n; i++) {
    res = scanf("%lld", a+i);
    sum[i%2] += a[i];
  }
  
  if ((n <= 2 && a[0] != a[1]) || (sum[0]%m != sum[1]%m)) {
    printf("-1\n");
    return 0;
  }
  
  if (n <= 2) {
    printf("%lld\n", a[0]+1LL);
    return 0;
  }
  
  powm1[0] = 1LL;
  for (int i = 1; i < n; i++) {
    powm1[i] = (((long long) (m-1))*powm1[i-1]) % mod_num;
  }
  
  for (int i = 0; i < n-1; i++) {
    ans += powm1[n-i-2]*a[i];
  }
  
  printf("%lld\n", ans%mod_num);
  return 0;
}