#include <bits/stdc++.h>
using namespace std;
#define modulo 1000000007
#define mod(mod_x) ((((long long)mod_x+modulo))%modulo)
#define Inf 2000000000000000000



int main(){
	
	int N;
	cin>>N;
	
	long long C;
	cin>>C;
	
	vector<long long> a(N),b(N);
	for(int i=0;i<N;i++)cin>>a[i]>>b[i];
	
	vector<vector<long long>> dp(N+1,vector<long long>(2,Inf));
	dp[0][1] = 0;
	
	for(int i=1;i<=N;i++){
		long long mini = Inf;
		long long t = 0;
		for(int j=i;j>=1;j--){
			t += C*abs(i-j)+a[i-1];
			dp[i][0] = min(dp[i][0],t+dp[j-1][1]);
		}
		dp[i][0] += b[i-1];
		t=0;
		dp[i][1] = dp[i][0];
		for(int j=i-1;j>=1;j--){
			t += C*abs(i-j);
			dp[i][1] = min(dp[i][1],t+dp[j][0]+a[j-1]*abs(i-j));
		}
	}
	
	cout<<dp.back().back()<<endl;
		
	
	return 0;
}