#include<iostream>
using namespace std;
#define N 100005
#define M 100005
int main() {
int a[N], b[M],t,x, n, m, sum = 0,i,sum2 = 0,j;
cin >> n >> m;
for (i = 1; i <= n; i++) {
cin >> a[i];
}
for (i = 1,x=1; i <= n,x<=n; i++,x++) {
b[x]= a[i];
}
i = n;
a[i+1] = m;
a[i + 2] = n;
for (j = 1; j < n+2; j++) {
for (i = 1; i <= n+2 - j; i++) {
if (a[i] > a[i + 1]) {
t = a[i];
a[i] = a[i + 1];
a[i + 1] = t;
}
}
}
a[0] = 0;
for (int i = 1; i <= n+2; i = i + 2) {
sum = sum + a[i] - a[i - 1];
}
b[0] = 0;
x = n;
b[x + 1] = m;
for (int i = 1; i <= x+1; i = i + 2) {
sum2 = sum2 + b[i] - b[i - 1];
}
cout << (sum > sum2 ? sum : sum2);
return 0;
}