#include <bits/stdc++.h>
using namespace std;
const int N=20000+5;
const int M=100+5;
int l,n,ans=0;
int x[M];
int main(){
    freopen ("device.in","r",stdin);
    freopen ("device.out","w",stdout);
    scanf("%d%d",&n,&l);l*=2;
    for (int i=1;i<=n;i++){
        scanf("%d",&x[i]);x[i]*=2;
    }
    sort(x+1,x+n+1);
    for (int i=1;i<=l;i++){
        int a=0,b=0,ok=0;
        for (int j=1;x[j]<i;j++)a=j;
        for (int j=n;x[j]>i;j--)b=j;
        if (a==0||b==0)continue;
        while(a>=1&&b<=n){
            if (x[a]+x[b]!=2*i){
                ok=1;break;
            }
            a--;b++;
        }
        if (ok==0)ans++;
    }
    printf("%d\n",ans);
    return 0;
}