#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;
#define ll long long
char getc(){char c=getchar();while ((c<'A'||c>'Z')&&(c<'a'||c>'z')&&(c<'0'||c>'9')) c=getchar();return c;}
int gcd(int n,int m){return m==0?n:gcd(m,n%m);}
int read()
{int x=0,f=1;char c=getchar();while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}while (c>='0'&&c<='9') x=(x<<1)+(x<<3)+(c^48),c=getchar();return x*f;
}
int a,b,c;
int main()
{a=read(),b=read(),c=read();cout<<b+min(c,a+b+1);return 0;
}

#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
using namespace std;
#define ll long long
#define N 200010
char getc(){char c=getchar();while ((c<'A'||c>'Z')&&(c<'a'||c>'z')&&(c<'0'||c>'9')) c=getchar();return c;}
int gcd(int n,int m){return m==0?n:gcd(m,n%m);}
int read()
{int x=0,f=1;char c=getchar();while (c<'0'||c>'9') {if (c=='-') f=-1;c=getchar();}while (c>='0'&&c<='9') x=(x<<1)+(x<<3)+(c^48),c=getchar();return x*f;
}
int n,m,a[N];
ll ans,f[2010][2010][2];
int main()
{m=read(),n=read();for (int i=1;i<=n;i++) a[i]=read();a[n+1]=m;memset(f,200,sizeof(f));f[1][n+1][0]=a[1],f[0][n][1]=m-a[n];for (int i=2;i<=n;i++)for (int j=0;j<=i;j++){if (j) f[j][n+1-(i-j)][0]=max(f[j-1][n+1-(i-j)][0]+a[j]-a[j-1],f[j-1][n+1-(i-j)][1]+a[j]+m-a[n+1-(i-j)]);if (j<i) f[j][n+1-(i-j)][1]=max(f[j][n+2-(i-j)][1]+a[n+2-(i-j)]-a[n+1-(i-j)],f[j][n+2-(i-j)][0]+a[j]+m-a[n+1-(i-j)]);}for (int i=1;i<=n;i++) ans=max(ans,f[i][i+1][0]),ans=max(ans,f[i][i+1][1]);cout<<ans;return 0;
}