#pragma GCC optimize ("O3")
#pragma GCC target ("avx2") // or sse4
#pragma GCC optimize("unroll-loops")

#include <bits/stdc++.h>
using namespace std;

template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}

//INSERT ABOVE HERE
using ull = long long;
const int MAX = 1e5+10;
ull as[MAX];
ull bs[MAX];

const ull INF = 4e18;

ull dp[MAX];
signed main(){
  cin.tie(0);
  ios::sync_with_stdio(0);

  int n,c;
  cin>>n>>c;
  for(int i=0;i<n;i++) cin>>as[i]>>bs[i];

  fill(dp,dp+MAX,INF);

  dp[0]=0;
  for(ull i=0;i<n;i++){
    ull a=as[i];
    ull cur=INF;

    for(ull p=0;p<=i;p++)
      chmin(cur,dp[p]-a*p+(p*(p-1)/2-i*p)*c);

    cur+=i*i*c;
    cur+=as[i]+bs[i];

    // [-, j]
    for(ull j=i;j<n;j++)
      chmin(dp[j+1],cur+a*j+(j*(j+1)/2-j*i)*c);
  }

  cout<<dp[n]<<endl;
  return 0;
}