RT.

#include<bits/stdc++.h>
#define ll long long
#define int ll
#define debug(x) cout<<#x<<":"<<x,puts("");
#define FOR(i,a,b) for(ll i=(a); i<=(b); ++i)
#define ROF(i,a,b) for(ll i=(a); i>=(b); --i)
//
//
//
using namespace std;
inline ll read() {
	ll f = 0, t = 0;
	char c = getchar();
	while (!isdigit(c)) t |= (c == '-'), c = getchar();
	while (isdigit(c)) f = (f << 3) + (f << 1) + c - 48, c = getchar();
	return t ? -f : f;
}
const int MX = 1e6 + 10;
struct {
	int w, h;
} a[MX];
int f[MX];
const int inf = 1e18;
struct {
	int sum, sum2, Sum;
} tree[MX * 4];
int lzy[MX * 4];
#define lw(x) (x<<1)
#define rw(x) (x<<1|1)
#define mid ((l+r)/2)
void push_down(int x) {
	lzy[lw(x)] = lzy[rw(x)] = lzy[x];
	tree[x].Sum -= tree[x].sum;
	tree[x].Sum += lzy[x];
	tree[x].sum = lzy[x];
	lzy[x] = 0;
}
void push_up(int x) {
	if (tree[lw(x)].Sum < tree[rw(x)].Sum) tree[x] = tree[lw(x)];
	else tree[x] = tree[rw(x)];
}
void change(int x, int l, int r, int ln, int rn, int w) {
	if (lzy[x]) push_down(x);
	if (ln <= l && r <= rn) {
		tree[x].Sum -= tree[x].sum;
		tree[x].Sum += w;
		tree[x].sum = w;
		lzy[x] = w;
		return;
	}
	if (lzy[x]) push_down(x);
	if (ln <= mid) change(lw(x), l, mid, ln, rn, w);
	if (mid < rn) change(rw(x), mid + 1, r, ln, rn, w);
	push_up(x);
}
void change_p(int x, int l, int r, int id, int w) {
	if (lzy[x]) push_down(x);
	if (l == r) {
		tree[x].sum2 = w;
		tree[x].Sum += w;
		return;
	}
	if (lzy[x]) push_down(x);
	if (id <= mid) change_p(lw(x), l, mid, id, w);
	else change_p(rw(x), mid + 1, r, id, w);
	push_up(x);
}
int find(int x, int l, int r, int ln, int rn) {
	if (lzy[x]) push_down(x);
	if (ln <= l && r <= rn) {
		return tree[x].Sum;
	}
	if (lzy[x]) push_down(x);
	int A = inf;
	if (ln <= mid) A = min(A, find(lw(x), l, mid, ln, rn));
	if (mid < rn) A = min(A, find(rw(x), mid + 1, r, ln, rn));
	push_up(x);
	return A;
}
int lst[MX];
signed main() {
	ios::sync_with_stdio(0), cout.tie(0);
	int n = read(), L = read();
	FOR(i, 2, n + 1) a[i].h = read(), a[i].w = read();
	FOR(i, 2, n + 1) f[i] = inf;
	FOR(i, 2, n + 1) lst[i] = -1;
	lst[1] = 1;
	a[1].h = inf;
	FOR(i, 2, n + 1) for (lst[i] = i - 1; a[lst[i]].h < a[i].h; lst[i] = lst[lst[i]]);
	int l = 2, sum = 0;
	f[1] = 0;
	FOR(i, 2, n + 1) {
		sum += a[i].w;
		while (sum > L) sum -= a[l].w, l++;
		change(1, 1, n, lst[i], i, a[i].h);
		f[i] = find(1, 1, n, l - 1, i - 1);
		change_p(1, 1, n, i, f[i]);
	}
	cout << f[n + 1];
	return 0;
}