#include<bits/stdc++.h>
using namespace std;
const int N=1e6+10;
int m,n,a[N],f[N],l[N],len;
int main()
{
    scanf("%d%d",&m,&n);
    for(int i=1;i<=n;i++)scanf("%d",&a[i]);
    sort(a+1,a+n+1);a[n+1]=m-1;a[0]=-0x3f3f3f3f;
    if(m-2==a[n]){while(a[n]==a[n-1]+1)n--;n--;}
    if(!n){printf("0\n");return 0;}
    len=1;f[1]=a[n+1]-a[n]-1,l[1]=1;
    for(int i=n-1;i>=1;i--)
    {
        if(a[i]==a[i+1]-1)l[len]++;
        else len++,f[len]=f[len-1]+a[i+1]-a[i]-1,l[len]=1;
    }
    if(a[n]==m-1){printf("%d\n",l[1]);return 0;}
    int ans=0,ss=0;
    for(int i=1;i<=len;i++)if(f[i]%2)ans^=l[i];
    if(!ans){printf("0\n");return 0;}
    for(int i=1;i<=len;i++)
        if(f[i]%2)
        {
            if(l[i]>=(l[i]^ans)&&(f[i+1]-1!=f[i]))ss++;
        }
        else
        {
            if(f[i]-1!=f[i-1])
            {
                if(l[i]>=ans)ss++;
            }
            else if(l[i]+l[i-1]>(l[i-1]^ans))ss++;
        }
    printf("%d\n",ss);
    return 0;
}