#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> PII;
const int maxn=66,maxm=3333,mod=998244353,INF=1e9;
#define fi first
#define se second
#define MP make_pair
#define PB push_back
#define lson o<<1,l,mid
#define rson o<<1|1,mid+1,r
#define FOR(i,a,b) for(int i=(a);i<=(b);i++)
#define ROF(i,a,b) for(int i=(a);i>=(b);i--)
#define MEM(x,v) memset(x,v,sizeof(x))
inline int read(){
	char ch=getchar();int x=0,f=0;
	while(ch<'0' || ch>'9') f|=ch=='-',ch=getchar();
	while(ch>='0' && ch<='9') x=x*10+ch-'0',ch=getchar();
	return f?-x:x;
}
inline int qpow(int a,int b){
	int ans=1;
	for(;b;b>>=1,a=1ll*a*a%mod) if(b&1) ans=1ll*ans*a%mod;
	return ans;
}
inline int qmo(int x){return x+(x>>31?mod:0);}
void clear(){

}
int n,a[maxn],v1,v2[maxn],m,L[maxn],R[maxn],lll[maxm],rrr[maxm],s[maxn];
ll F[maxn][maxn][maxm],G[maxn][maxn][maxm],H[maxn][maxn][maxm];
// all,down,up
inline ll& f(int l,int r,int i,int j){return F[l][r][L[i]+j];}
inline ll& g(int l,int r,int i,int j){return G[l][r][L[i]+j];}
inline ll& h(int l,int r,int i,int j){return H[l][r][L[i]+j];}
inline int sz(int l,int r){return l>r?0:s[r]-s[l-1];}
inline int szf(int l,int r,int i,int j){return j>a[i] || j<1?0:sz(l,r);}
inline int szg(int l,int r,int i,int j){return j>a[i] || j<1?0:sz(l,r)-a[i]+j;}
inline int szh(int l,int r,int i,int j){return j>a[i] || j<1?0:sz(l,r)+a[i]-j+1;}
inline int& lft(int i,int j){return lll[L[i]+j];}
inline int& rig(int i,int j){return rrr[L[i]+j];}
inline ll func(int c,int x){return 1ll*x*(m-x)*c;}
inline void chkmin(ll &x,ll y){if(y<x) x=y;}
void mian(){
	MEM(F,0x3f);MEM(G,0x3f);MEM(H,0x3f);
	n=read();v1=read();
	FOR(i,1,n){
		a[i]=read();v2[i]=read();
		s[i]=s[i-1]+a[i];
		L[i]=R[i-1]+1;R[i]=L[i]+a[i]+1;
		m+=a[i];
	}
	FOR(i,1,n){
		ll sum=0;
		FOR(j,1,a[i]){
			FOR(k,1,n+1) h(k,k-1,i,a[i]-j+1)=sum;
			sum=sum+func(v2[i],j);
		}
		FOR(j,1,a[i]){
			ROF(k,i-1,1) if(a[k]>=j){lft(i,j)=k;break;}
			rig(i,j)=n+1;
			FOR(k,i+1,n) if(a[k]>=j){rig(i,j)=k;break;}
		}
		g(i,i,i,0)=0;
		FOR(k,1,n+1) h(k,k-1,i,a[i]+1)=0;
	}
	ROF(l,n,1) FOR(r,l,n){
		FOR(i,l,r) FOR(j,1,a[i]){
			int prv=lft(i,j),nxt=rig(i,j);
			chkmin(g(l,r,i,j),g(l,r,i,j-1)+func(v2[i],szg(l,r,i,j-1)));
			if(prv>=l){
				FOR(k,prv,i-1) chkmin(g(l,r,i,j),g(k+1,r,i,j-1)+f(l,k,prv,j)+func(v2[i],szg(k+1,r,i,j-1))+func(v1,szf(l,k,prv,j)));
			}
			if(nxt<=r){
				FOR(k,i+1,nxt) chkmin(g(l,r,i,j),g(l,k-1,i,j-1)+f(k,r,nxt,j)+func(v2[i],szg(l,k-1,i,j-1))+func(v1,szf(k,r,nxt,j)));
				if(prv>=l){
					FOR(k,prv,i-1) FOR(kk,i+1,nxt) chkmin(g(l,r,i,j),g(k+1,kk-1,i,j-1)+f(l,k,prv,j)+f(kk,r,nxt,j)+func(v2[i],szg(k+1,kk-1,i,j-1))+func(v1,szf(l,k,prv,j))+func(v1,szf(kk,r,nxt,j)));
				}
			}
		}
		FOR(i,l,r){
			FOR(j,1,a[i]) FOR(k,l-1,i-1) chkmin(f(l,r,i,j),g(k+1,r,i,j)+h(l,k,i,j));
			FOR(j,1,a[i]) FOR(k,i+1,r+1) chkmin(f(l,r,i,j),g(l,k-1,i,j)+h(k,r,i,j));
		}
		FOR(i,1,l-1) ROF(j,a[i],1){
			int nxt=rig(i,j);
			chkmin(h(l,r,i,j),h(l,r,i,j+1)+func(v2[i],szh(l,r,i,j+1)));
			if(nxt>=l && nxt<=r) FOR(k,nxt,r) chkmin(h(l,r,i,j),h(k+1,r,i,j+1)+f(l,k,nxt,j)+func(v2[i],szh(k+1,r,i,j+1))+func(v1,szf(l,k,nxt,j)));
		}
		FOR(i,r+1,n) ROF(j,a[i],1){
			int prv=lft(i,j);
			chkmin(h(l,r,i,j),h(l,r,i,j+1)+func(v2[i],szh(l,r,i,j+1)));
			if(prv>=l && prv<=r) FOR(k,l,prv) chkmin(h(l,r,i,j),h(l,k-1,i,j+1)+f(k,r,prv,j)+func(v2[i],szh(l,k-1,i,j+1))+func(v1,szf(k,r,prv,j)));
		}
	}
	printf("%lld\n",f(1,n,1,a[1]));
}
int main(){
	int T=1;
//	T=read();
	while(T--) mian();
}