#include <bits/stdc++.h>
using namespace std;
const int MOD = (int)1e9 + 7;
const int MOD2 = 1007681537;
const int INF = (int)1e9;
const long long LINF = (long long)1e18;
const long double PI = acos((long double)-1);
const long double EPS = 1e-9;
inline long long gcd(long long a, long long b) {
  long long r;
  while (b) {
    r = a % b;
    a = b;
    b = r;
  }
  return a;
}
inline long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
inline long long fpow(long long n, long long k, int p = MOD) {
  long long r = 1;
  for (; k; k >>= 1) {
    if (k & 1) r = r * n % p;
    n = n * n % p;
  }
  return r;
}
template <class T>
inline int chkmin(T& a, const T& val) {
  return val < a ? a = val, 1 : 0;
}
template <class T>
inline int chkmax(T& a, const T& val) {
  return a < val ? a = val, 1 : 0;
}
inline long long isqrt(long long k) {
  long long r = sqrt(k) + 1;
  while (r * r > k) r--;
  return r;
}
inline long long icbrt(long long k) {
  long long r = cbrt(k) + 1;
  while (r * r * r > k) r--;
  return r;
}
inline void addmod(int& a, int val, int p = MOD) {
  if ((a = (a + val)) >= p) a -= p;
}
inline void submod(int& a, int val, int p = MOD) {
  if ((a = (a - val)) < 0) a += p;
}
inline int mult(int a, int b, int p = MOD) { return (long long)a * b % p; }
inline int inv(int a, int p = MOD) { return fpow(a, p - 2, p); }
inline int sign(long double x) { return x < -EPS ? -1 : x > +EPS; }
inline int sign(long double x, long double y) { return sign(x - y); }
const int maxn = 4e5 + 5;
int n;
long long c;
long long a[maxn];
int l[maxn];
long long dp[maxn];
long long sum[maxn];
long long st[maxn << 1];
void upd(int p, long long val) {
  for (st[p += maxn] = val; p > 1;)
    p >>= 1, st[p] = max(st[p << 1], st[p << 1 | 1]);
}
long long query(int l, int r) {
  long long res = -LINF;
  for (l += maxn, r += maxn + 1; l < r; l >>= 1, r >>= 1) {
    if (l & 1) chkmax(res, st[l++]);
    if (r & 1) chkmax(res, st[--r]);
  }
  return res;
}
void solve() {
  fill_n(st, maxn << 1, -LINF);
  cin >> n >> c;
  for (int i = (1); i < (n + n); i++) cin >> a[i];
  partial_sum(a, a + n + n, a);
  int ptr = 1;
  for (int i = (1); i < (n + 1); i++) {
    while (ptr < i && a[i + i - 1] - a[ptr + ptr - 2] > c) ptr++;
    l[i] = ptr;
  }
  dp[1] = sum[1] = min(a[1], c);
  for (int i = (0); i < (1 + 1); i++) {
    upd(i, sum[i] - a[i + i]);
  }
  for (int i = (2); i < (n + 1); i++) {
    dp[i] = (a[i + i - 1] - sum[i - 1]) + query(l[i] - 1, i - 2);
    if (l[i] >= 2) {
      chkmax(dp[i], c - (sum[i - 1] - sum[l[i] - 2]));
    }
    chkmax(dp[i], min(a[i + i - 1] - a[i + i - 2], c));
    sum[i] = sum[i - 1] + dp[i];
    upd(i, sum[i] - a[i + i]);
  }
  cout << sum[n] << "\n";
}
int main() {
  int JUDGE_ONLINE = 1;
  if (fopen("in.txt", "r")) {
    JUDGE_ONLINE = 0;
    assert(freopen("in.txt", "r", stdin));
  } else {
    ios_base::sync_with_stdio(0), cin.tie(0);
  }
  solve();
  if (!JUDGE_ONLINE) {
  }
  return 0;
}
