#include<bits/stdc++.h>
#define Gc() getchar() 
#define Me(x,y) memset(x,y,sizeof(x))
#define Mc(x,y) memcpy(x,y,sizeof(x))
#define d(x,y) ((m)*(x-1)+(y))
#define R(n) (rnd()%(n)+1)
#define Pc(x) putchar(x)
#define LB lower_bound
#define UB upper_bound
#define fi first
#define se second
using namespace std;using ll=long long;using db=double;using lb=long db;using ui=unsigned;using ull=unsigned long long;using pii=pair<int,int>;using LL=__int128;
const int N=2e5+5,M=N*100+5,K=600+5,mod=1e9+7,Mod=mod-1;const db eps=1e-9;const int INF=1e9+7;mt19937 rnd(263082);
int n,m,A[N],B[N],f[N];
int CK(int mid){
	fill(f+1,f+n+1,0);int i;
	for(int i=1;i<=n;i++) {
		if(B[i]-A[i]>mid) f[1]++,f[A[i]]--,f[B[i]]++;
		if(n-(B[i]-A[i])>mid) f[A[i]]++,f[B[i]]--;
	}
	for(i=1;i<=n;i++) f[i]+=f[i-1];
	for(i=1;i<=n;i++) if(!f[i]) return 1;
	return 0;
}
void Solve(){
	int i,j;if(scanf("%d%d",&n,&m)==-1) exit(0);
	for(i=1;i<=m;i++) scanf("%d%d",&A[i],&B[i]),A[i]>B[i]&&(swap(A[i],B[i]),0);
	int l=-1,r=n+1,mid;while(l+1<r) mid=l+r>>1,(CK(mid)?r:l)=mid;
	printf("%d\n",r);
}
int main(){
	int t=1;
	// scanf("%d",&t);
	while(1) Solve();
	cerr<<clock()*1.0/CLOCKS_PER_SEC<<'\n';
}