#include<iostream>
#include<algorithm>
#include<cstring>
#include<cstdio>
#include<bitset>
#include<cmath>
#include<ctime>
#include<queue>
#include<map>
#include<set>

#define fi first
#define se second
#define max Max
#define min Min
#define abs Abs
#define lc (x<<1)
#define rc (x<<1|1)
#define mid ((l+r)>>1)
#define pb(x) push_back(x)
#define lowbit(x) (x&(-x))
#define fan(x) (((x-1)^1)+1)
#define mp(x,y) make_pair(x,y)
#define clr(f,n) memset(f,0,sizeof(int)*(n))
#define cpy(f,g,n) memcpy(f,g,sizeof(int)*(n))
#define INF 0x3f3f3f3f

using namespace std;

inline int read()
{
	int ans=0,f=1;
	char c=getchar();
	while(c>'9'||c<'0'){if(c=='-')f=-1;c=getchar();}
	while(c>='0'&&c<='9'){ans=(ans<<1)+(ans<<3)+c-'0';c=getchar();}
	return ans*f;
}

inline void write(int x)
{
	if(x<0) putchar('-'),x=-x;
	if(x/10) write(x/10);
	putchar((char)(x%10)+'0');
}

template<typename T>inline T Abs(T a){return a>0?a:-a;};
template<typename T,typename TT>inline T Min(T a,TT b){return a<b?a:b;}
template<typename T,typename TT> inline T Max(T a,TT b){return a<b?b:a;}

const int N=2e5+5;
int n,m,b[N];

struct Node
{
	int x,y;
}a[N];

inline void add(int l,int r)
{
	if(l>r) return;
	b[l]++;b[r+1]--;
}

inline bool check(int x)
{
	for(int i=1;i<=m;++i)
	{
		int p1=a[i].y-a[i].x;
		int p2=a[i].x+n-a[i].y;
		if(p1<=x&&p2<=x) continue;
		if(p1>x&&p2>x) return 0;
		if(p1>x) add(1,a[i].x-1),add(a[i].y,n);
		if(p2>x) add(a[i].x,a[i].y-1);
	}
	int flag=0;
	for(int i=1;i<=n;++i)
	{
		b[i]+=b[i-1];
		if(b[i]==0) flag=1;
	}
	for(int i=1;i<=n+1;++i)
		b[i]=0;
	return flag;
}

signed main()
{
	while(scanf("%d%d",&n,&m)!=EOF)
	{
		for(int i=1;i<=m;++i)
		{
			a[i].x=read();
			a[i].y=read();
			if(a[i].x>a[i].y)
				swap(a[i].x,a[i].y);
		}
		int l=0,r=n+2,ps=n+1;
		while(l<=r)
		{
			if(check(mid))
				ps=mid,r=mid-1;
			else l=mid+1;
		}
		write(ps);puts("");
	}
	return 0;
}