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#include<bits/stdc++.h>

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define FORD(i, a, b) for(int i = (a); i >= (b); --i)
#define VAR(v, i) __typeof(i) v=(i)
#define FORE(i, c) for(VAR(i, (c).begin()); i != (c).end(); ++i)


#define VI vector<int>
#define PII pair<int,int>
#define st first
#define nd second
#define mp make_pair
#define pb push_back
#define lint long long int


#define debug(x) {cerr <<#x <<" = " <<x <<endl; }
#define debug2(x,y) {cerr <<#x <<" = " <<x << ", "<<#y<<" = "<< y <<endl; }
#define debug3(x,y,z) {cerr <<#x <<" = " <<x << ", "<<#y<<" = "<< y << ", " << #z << " = " << z <<endl; }
#define debugv(x) {{cerr <<#x <<" = "; FORE(itt, (x)) cerr <<*itt <<", "; cerr <<endl; }}
#define debugt(t,n) {{cerr <<#t <<" = "; FOR(it,0,(n)) cerr <<t[it] <<", "; cerr <<endl; }}


#define make( x) int (x); scanf("%d",&(x));
#define make2( x, y) int (x), (y); scanf("%d%d",&(x),&(y));
#define make3(x, y, z) int (x), (y), (z); scanf("%d%d%d",&(x),&(y),&(z));
#define make4(x, y, z, t) int (x), (y), (z), (t); scanf("%d%d%d%d",&(x),&(y),&(z),&(t));
#define IOS ios_base::sync_with_stdio(0)
#define HEAP priority_queue


#define read( x) scanf("%d",&(x));
#define read2( x, y) scanf("%d%d",&(x),&(y));
#define read3(x, y, z) scanf("%d%d%d",&(x),&(y),&(z));
#define read4(x, y, z, t) scanf("%d%d%d%d",&(x),&(y),&(z),&(t));


using namespace std;

lint by_color[500005]; 
lint best_so_far = 0;

int main() {
	int h = -1;
	vector<pair<int, lint> > cur_h;

	make2(n, c);
	FOR(i,0,n) {
		make2(a, b);
		if (a > h) {
			FOR(j,0,cur_h.size()) {
				int col = cur_h[j].st;
				lint wyn = cur_h[j].nd; 
				by_color[col] = max(by_color[col], wyn);
				best_so_far = max(best_so_far, wyn);
			}
			cur_h.clear();
			h = a ;
		}
		lint res = max(a + best_so_far - c, a + by_color[b]);	
		cur_h.pb({b,res});
	}

			FOR(j,0,cur_h.size()) {
				int col = cur_h[j].st;
				lint wyn = cur_h[j].nd; 
				by_color[col] = max(by_color[col], wyn);
				best_so_far = max(best_so_far, wyn);
			}

	cout << best_so_far << endl;
}