#include<iostream>#include<cstdio>#include<string>#include<cstring>#include<algorithm>#include<cmath>using namespace std;const int maxn = 100005;int n,m,cnt,d[maxn],h[maxn];int main(){    freopen("note.in","r",stdin);    freopen("note.out","w",stdout);    cin>>n>>m;    for(int i = 1;i <= m;i++){        scanf("%d%d",&d[i],&h[i]);    }    int ans = max(h[m] + n - d[m],h[1] + d[1] - 1);    int hg,lw,div,mid;    for(int i = 2;i <= m;i++){        div = h[i] - h[i-1];        if(div < 0) div = -div;        if(div > d[i]-d[i-1]){            cout<<"IMPOSSIBLE";            return 0;        }        mid = (h[i] + h[i-1] + d[i] - d[i-1]) >> 1;        ans = max(ans,mid);    }    cout<<ans;    return 0;}#include<cstdio>#include<iostream>#include<algorithm>#include<cstring>#include<cmath>#include<vector>#include<queue>#include<map>#include<set>#include<stack>#include<cstdlib>#include<string>#include<bitset>#define INF 200000000#define N 100005#define fi first#define se second#define debug(x) cout<<#x<<"="<<x<<endl#define MP(x,y) make_pair(x,y)using namespace std;typedef long long LL;typedef pair<int,int> pii;int a[N],b[N];int main(){    int n,m,ans=0,flag,t,l,r,mid,x,now,i;    freopen("note.in","r",stdin);    freopen("note.out","w",stdout);    cin>>n>>m;    for(i=1;i<=m;i++)    {        scanf("%d%d",&a[i],&b[i]);        if(i==1)            ans=b[i]+a[i]-1;        else        {            t=a[i]-a[i-1];            x=b[i]-b[i-1];            if(abs(x)>t)            {                printf("IMPOSSIBLE\n");                return 0;            }            x=abs(x);            ans=max(ans,max(b[i],b[i-1]));            l=0,r=INF;            now=0;            while(l<=r)            {                mid=(l+r)>>1;                if(mid+abs(mid-x)<=t)                    now=mid,l=mid+1;                else r=mid-1;            }            ans=max(ans,min(b[i],b[i-1])+now);        }        if(i==m)            ans=max(ans,b[i]+n-a[i]);    }    cout<<ans<<endl;    return 0;}// davidlee1999WTK 2015/// srO myk Orz//ios::sync_with_stdio(false);/*100 110 10*/