#include <cstdio>
#include <algorithm>
#include <limits>
#define repi(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,a) repi(i,0,a)
#define all(a) (a).begin(), (a).end()

constexpr int MAX_N = 100;
using ll = long long;
constexpr ll INF = std::numeric_limits<ll>::max()>>2;

ll N, D;
ll C[2][MAX_N];
ll dp[MAX_N+1][2];

void chmax( ll &a, ll b )
{ a = std::max( a, b ); }

int main()
{
  scanf( "%lld%lld", &N, &D );
  rep( i, N )
    scanf( "%lld%lld", C[0]+i, C[1]+i );

  dp[0][0] = 0;
  dp[0][1] = -INF;
  rep( i, N ) rep( j, 2 )
  {
    chmax( dp[i+1][j], dp[i][!j]-D+C[j][i] );
    chmax( dp[i+1][j], dp[i][j]+C[j][i] );
  }

  printf( "%lld\n", std::max( dp[N][0], dp[N][1] ) );

  return 0;
}