//2020.3.28 by ljz
//email 573902690@qq.com
//if you find any bug in my code
//please tell me
#include<bits/stdc++.h>
//#include<ext/pb_ds/tree_policy.hpp>
//#include<ext/pb_ds/assoc_container.hpp>
using namespace std;
//using namespace __gnu_pbds;
//using namespace __gnu_cxx;
#define res register int
#define LL long long
#define inf 0x3f3f3f3f
#define INF 0x3f3f3f3f3f3f3f
#define unl __int128
#define eps 5.6e-8
#define RG register
#define db double
#define pc(x) __builtin_popcount(x)
#define ctz(x) __builtin_ctz(x)
//#define pc(x) __builtin_popcountll(x)
typedef pair<int,int> Pair;
#define mp make_pair
#define fi first
#define se second
#define pi acos(-1.0)
#define pb push_back
#define ull unsigned LL
#define lowbit(x) (x&-x)
#define gc getchar
#define kcz 998244353
//template <class T>using Tree=tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update>;
// inline char gc() {
//    static char buf[100000],*p1,*p2;
//    return p1==p2&&(p2=(p1=buf)+fread(buf,1,100000,stdin),p1==p2)?EOF:*p1++;
// }
inline int read() {
    res s=0,ch=gc();
    while(ch<'0'||ch>'9')ch=gc();
    while(ch>='0'&&ch<='9')s=s*10+ch-'0',ch=gc();
    return s;
}
// char sr[1<<21],z[20];
// int C=-1,Z=0;
// inline void Ot(){fwrite(sr,1,C+1,stdout),C=-1;}
// inline void print(res x){
//    if(C>1<<20)Ot();if(x<0)sr[++C]='-',x=-x;
//    while(z[++Z]=x%10+48,x/=10);
//    while(sr[++C]=z[Z],--Z);
// }
//inline int read() {
//    res s=0,ch=gc(),w=1;
//    while(ch<'0'||ch>'9'){if(ch=='-')w=-1;ch=gc();}
//    while(ch>='0'&&ch<='9')s=s*10+ch-'0',ch=gc();
//    return s*w;
//}
//inline LL Read() {
//    RG LL s=0;
//    res ch=gc();
//    while(ch<'0'||ch>'9')ch=gc();
//    while(ch>='0'&&ch<='9')s=s*10+ch-'0',ch=gc();
//    return s;
//}
//inline LL Read() {
//    RG LL s=0;
//    res ch=gc(),w=1;
//    while(ch<'0'||ch>'9'){if(ch=='-')w=-1;ch=gc();}
//    while(ch>='0'&&ch<='9')s=s*10+ch-'0',ch=gc();
//    return s*w;
//}
//inline void write(RG unl x){
//    if(x>10)write(x/10);
//    putchar(int(x%10)+'0');
//}
//inline void swap(res &x,res &y) {
//    x^=y^=x^=y;
//}
//inline void add(res &x,const res &y){
//    x+=y,x>=kcz?x-=kcz:(x<0?x+=kcz:1);
//}
//inline int Add(const res &x,const res &y){
//    return x+y>=kcz?x+y-kcz:(x+y<0?x+y+kcz:x+y);
//}
//inline int mul(const res &x,const res &y){
//    return int(1LL*x*y%kcz);
//}
//inline int mul(const res &x,const res &y,const res &d){
//    return int(1LL*x*y/d%kcz);
//}
//inline int qpow(res x,res y){
//    res ret=1;
//    while(y){
//        if(y&1)ret=mul(ret,x);
//        x=mul(x,x),y>>=1;
//    }
//    return ret;
//}
//mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
//clock_t start=clock();
//inline void ck(){
//    if(1.0*(clock()-start)/CLOCKS_PER_SEC>0.1)exit(0);
//}
namespace MAIN{
    int a[200000+10];
    inline void MAIN(){
        res k=read(),n=read();
        res ans=inf;
        for(res i=1;i<=n;i++)a[i]=read();
        for(res i=2;i<n;i++)ans=min({ans,a[i]-a[i+1]+k,a[i-1]-a[i]+k});
        ans=min({ans,a[n]-a[1],a[1]+k-a[n]+a[n]-a[2],a[1]+k-a[n]+a[n-1]-a[1]});
        printf("%d\n",ans);
    }
}
int main(){
//    srand(19260817);
    // freopen("tracing.in","r",stdin);
    // freopen("tracing.out","w",stdout);
    MAIN::MAIN();
//    Ot();
    return 0;
}

Measured memory_bytes: 1024