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

signed main() {
	int n,k;cin>>n>>k;
	vector<int> tav(n),v(n),w(n);
	for(int i = 0;i<n;i++){
		cin>>v[i]>>w[i];
		if(i>0)tav[i]=tav[i-1]+v[i-1];
	}
	tav.push_back(tav[n-1]+v[n-1]);
	v.push_back(0);
	w.push_back(0);
	deque<pair<int,int>> q;
	int gas=0,cost=0,j=0,i=0;
	while(i<=n){
		j=max(i,j);
		if(q.front().second<i)q.pop_front();
		while(j<=n&&tav[j]<gas+tav[i]){
			while(!q.empty()&&q.back().first>=w[j])q.pop_back();
			q.push_back({w[j],j});
			j++;
		}
		if(j==n){
			break;
		}
		int kell=tav[j]-tav[i]-gas;
		gas-=tav[q.front().second]-tav[i];
		i=q.front().second;
		cost+=w[i];
		gas+=kell;
	}
	cout<<cost<<endl;
}