// what is matter? never mind. 
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,sse4,popcnt,abm,mmx,avx,avx2") 
#include<bits/stdc++.h>
#define For(i,a,b) for(int i=(a);i<=(b);++i)
#define Rep(i,a,b) for(int i=(a);i>=(b);--i)
#define ll long long
#define int long long
#define ull unsigned long long
#define SZ(x) ((int)((x).size()))
#define ALL(x) (x).begin(),(x).end()
//#define double long double
using namespace std;
inline int read()
{
    char c=getchar();int x=0;bool f=0;
    for(;!isdigit(c);c=getchar())f^=!(c^45);
    for(;isdigit(c);c=getchar())x=(x<<1)+(x<<3)+(c^48);
    if(f)x=-x;return x;
}

#define fi first
#define se second
#define pb push_back
#define mkp make_pair
typedef pair<int,int>pii;
typedef vector<int>vi;

#define maxn 300005
#define inf 0x3f3f3f3f3f3f3f3f

int n,V;
struct node{
	int h,c;
}a[maxn];

int t[maxn],m,sum[maxn],mn[maxn];
#define N 45
int f[N][N],tf[N][N];
vi buc[maxn];

void chkmin(int&x,int y){
	x=min(x,y);
}

signed main()
{
	n=read(),V=read();
	For(i,1,n)a[i].h=read(),a[i].c=read();
	sort(a+1,a+n+1,[&](node x,node y){
		return x.c<y.c;
	});
	For(i,1,n) For(j,0,n-1) t[++m]=a[i].h+j;
	sort(t+1,t+m+1),m=unique(t+1,t+m+1)-t-1;
	#define Val(x) lower_bound(t+1,t+m+1,x)-t
	For(i,1,n) {
		buc[Val(a[i].h)].pb(i);
	}
	memset(f,63,sizeof f);
	
	mn[0]=inf;
	For(i,1,m) {
		sum[i]=sum[i-1]+buc[i].size();
		mn[i]=mn[i-1];
		for(int x:buc[i]) mn[i]=min(mn[i],x);
	}
	
	For(i,1,m) {
		int cnt=buc[i].size();
	//	cout<<"i: "<<i<<" "<<t[i]<<" "<<cnt<<"\n";
		memset(tf,63,sizeof tf);
		int id=mn[i];
		// jiekou meijieshangde min
		For(x,0,n)For(y,0,n) if(f[x][y]<inf) {
		//	cout<<"x,y,z "<<x<<" "<<y<<" "<<z<<" "<<f[x][y][z]<<"\n";
			int nx=x;
			int ny=y+cnt;
			int tmp=min(nx,ny);
			int add=0;
			if(tmp) ny-=tmp,add+=tmp*V*t[i];
			int vals=t[i]*V+a[id].c;
			For(p,0,ny) chkmin(tf[nx+p][ny-p],f[x][y]+vals*p+add);
		}
		if(cnt){
			int u=buc[i][0];
			assert(a[u].h==t[i]);
			chkmin(tf[1][sum[i]-1],t[i]*V);
		}
		swap(tf,f);
	}
	int res=inf;
	For(x,0,n) res=min(res,f[x][0]);
	int ss=0;
	For(i,1,n) res-=a[i].h*V,ss+=a[i].h*V;
//	cout<<"SS "<<ss<<"\n";
	cout<<res;
	return 0;
}
/*

*/