#pragma GCC optimize ( "O3" )
#pragma GCC target ( "avx" )
#include <bits/stdc++.h>
using namespace std;

using ll = long long;

#define TYPE_OF( VAR ) remove_const<remove_reference<decltype( VAR )>::type >::type
#define UNTIE ios_base::sync_with_stdio( false ); cin.tie( nullptr ) 
#define CEXPR( LL , BOUND , VALUE ) constexpr const LL BOUND = VALUE 
#define CIN( LL , A ) LL A; cin >> A 
#define ASSERT( A , MIN , MAX ) assert( MIN <= A && A <= MAX ) 
#define CIN_ASSERT( A , MIN , MAX ) CIN( TYPE_OF( MAX ) , A ); ASSERT( A , MIN , MAX ) 
#define FOR( VAR , INITIAL , FINAL_PLUS_ONE ) for( TYPE_OF( FINAL_PLUS_ONE ) VAR = INITIAL ; VAR < FINAL_PLUS_ONE ; VAR ++ ) 
#define QUIT return 0 
#define RETURN( ANSWER ) cout << ( ANSWER ) << "\n"; QUIT 

#define POWER_MOD( ANSWER , ARGUMENT , EXPONENT , MODULO )		\
  TYPE_OF( ARGUMENT ) ANSWER{ 1 };					\
  {									\
    TYPE_OF( ARGUMENT ) ARGUMENT_FOR_SQUARE_FOR_POWER = ( ARGUMENT ) % MODULO; \
    TYPE_OF( EXPONENT ) EXPONENT_FOR_SQUARE_FOR_POWER = ( EXPONENT );	\
    while( EXPONENT_FOR_SQUARE_FOR_POWER != 0 ){			\
      if( EXPONENT_FOR_SQUARE_FOR_POWER % 2 == 1 ){			\
	ANSWER = ( ANSWER * ARGUMENT_FOR_SQUARE_FOR_POWER ) % MODULO;	\
      }									\
      ARGUMENT_FOR_SQUARE_FOR_POWER = ( ARGUMENT_FOR_SQUARE_FOR_POWER * ARGUMENT_FOR_SQUARE_FOR_POWER ) % MODULO; \
      EXPONENT_FOR_SQUARE_FOR_POWER /= 2;				\
    }									\
  }									\



int main()
{
  UNTIE;
  CEXPR( int , bound_N , 200000 );
  CIN_ASSERT( N , 2 , bound_N );
  CEXPR( ll , bound_M , 1000000 );
  CIN_ASSERT( M , 3 , bound_M );
  ll A[bound_N];
  ll M_minus_2 = M - 2;
  ll res = 0;
  FOR( i , 0 , N ){
    CIN_ASSERT( Ai , 0 , M_minus_2 );
    res = ( Ai + ( M - res ) ) % M;
    A[i] = Ai;
  }
  if( res != 0 ){
    RETURN( -1 );
  }
  ll answer = 0;
  ll M_minus_1 = M - 1;
  CEXPR( ll , P , 998244353 );
  FOR( i , 0 , N ){
    // やったーp進だー！
    answer = ( ( answer * M_minus_1 ) + A[i] ) % P;
  }
  POWER_MOD( M_inv , M , P - 2 , P );
  ( ( answer *= M_inv ) += 1 ) %= P;
  RETURN( answer );
}