#include<bits/stdc++.h>
using namespace std;
int pn[1005];
int n,m,a;
int fe[105];
int zy(int x)
{
    if(x==0) return 99999;
    int s=0,p=0,q=0;
    for(int i=x+1;i<=n;i++)
    {
        if(pn[i]==0) s++;
        if(pn[i]==1) {q=i;break;}
        
    }
    for(int i=x-1;i>=1;i--)
    {
        if(pn[i]==0) s++;
        if(pn[i]==1) {p=i;break;}
    }
    return abs(x-(p+q)/2);
}
int rr(int x)
{
    if(x==0) return 99999;
    int s=0;
    for(int i=x+1;i<=n;i++)
    {
        if(pn[i]==0) s++;
        if(pn[i]==1) {;break;}
        
    }
    for(int i=x-1;i>=1;i--)
    {
        if(pn[i]==0) s++;
        if(pn[i]==1) {break;}
    }
    return s;
}
int main()
{
    freopen("prison.in","r",stdin);
    freopen("prison.out","w",stdout);
    memset(pn,0,sizeof(pn));
    cin>>n>>m;
    for(int i=1;i<=m;i++)
    {
        cin>>fe[i];
    }
    int ans=0;
    int free=99999;
   for(int j=1;j<=m;j++)
   { 
        for(int i=1;i<=m;i++)
        {
            if(zy(fe[i])<free)
            {
                free=rr(fe[i]);
                a=i;
            }    
        }
        pn[fe[a]]=1;
        fe[a]=0;
        ans+=free;
    }
    cout<<ans;
}