#include<cstdio> #include<cstdlib> #include<cstring> #include<cmath> #include<ctime> #include<cctype> #include<climits> #include<iostream> #include<string> #include<vector> #include<map> #include<list> #include<queue> #include<deque> #include<algorithm> #include<numeric> #include<utility> #include<complex> #include<memory> #include<functional> #include<set> using namespace std; #define ALL(g) (g).begin(),(g).end() #define REP(i, x, n) for(int i = x; i < n; i++) #define rep(i,n) REP(i,0,n) #define F(i,j,k) fill(i[0],i[0]+j*j,k) #define P(p) cout<<(p)<<endl; #define EXIST(s,e) ((s).find(e)!=(s).end()) #define INF 1<<25 #define pb push_back typedef vector<int> vi; typedef vector<long long> vl; typedef vector<double> vd; typedef pair<int,int> pii; typedef pair<long,long> pll; typedef long long ll; int N,D; int in[101][2],dp[101][2];//[day][place] /*void dfs(int day,int yen,int place){ if(day==N) return ; if(place==0) yen+=t[day]; else yen+=k[day]; dp[day][place]=max( //stay dfs(day+1,yen,place); //move dfs(day+1,yen,1-place); } */ int main(){ cin>>N>>D; rep(i,N) cin>>in[i][0]>>in[i][1]; dp[0][1]=-D; rep(i,N){ rep(j,2){ //stay dp[i+1][j]=max(dp[i+1][j],dp[i][j]+in[i][j]); //move dp[i+1][1-j]=max(dp[i+1][1-j],dp[i][j]+in[i][1-j]-D); } } int ans=max(dp[N][0],dp[N][1]); cout<<ans<<endl; return 0; }