#include <iostream>

#include <sstream>

#include <algorithm>

#include <cstdio>

#include <cmath>

#include <set>

#include <map>

#include <queue>

#include <string>

#include <cstring>

#include <bitset>

#include <functional>

#include <random>

#define REP(i,a,n) for(int i=a;i<=n;++i)

#define PER(i,a,n) for(int i=n;i>=a;--i)

#define hr putchar(10)

#define pb push_back

#define lc (o<<1)

#define rc (lc|1)

#define mid ((l+r)>>1)

#define ls lc,l,mid

#define rs rc,mid+1,r

#define x first

#define y second

#define io std::ios::sync_with_stdio(false)

#define endl '\n'

#define DB(a) ({REP(__i,1,n) cout<<a[__i]<<',';hr;})

using namespace std;

typedef long long ll;

typedef pair<int,int> pii;

const int P = 1e9+7, INF = 0x3f3f3f3f;

ll gcd(ll a,ll b) {return b?gcd(b,a%b):a;}

ll qpow(ll a,ll n) {ll r=1%P;for (a%=P;n;a=a*a%P,n>>=1)if(n&1)r=r*a%P;return r;}

ll inv(ll x){return x<=1?1:inv(P%x)*(P-P/x)%P;}

inline int rd() {int x=0;char p=getchar();while(p<'0'||p>'9')p=getchar();while(p>='0'&&p<='9')x=x*10+p-'0',p=getchar();return x;}

//head

const int N = 1e6+50;

int n,m,ok,vis[N],l[N],d[N],mi[N],ID[N],cur[N];

vector<int> g[N];

pii f[N],A,B;

void dfs(int x, int c) {

	vis[x] = c;

	if (c) {

		A.x = min(A.x,l[x]);

		A.y = max(A.y,l[x]);

		B.x = min(B.x,d[x]);

		B.y = max(B.y,d[x]);

	}

	else {

		A.x = min(A.x,d[x]);

		A.y = max(A.y,d[x]);

		B.x = min(B.x,l[x]);

		B.y = max(B.y,l[x]);

	}

	for (int y:g[x]) {

		if (vis[y]<0) dfs(y,c^1);

		else if (vis[y]==c) ok=0;

	}

}

void work() {

	scanf("%d%d",&n,&m);

	REP(i,1,n) vis[i]=-1,g[i].clear();

	REP(i,1,m) {

		int u, v;

		scanf("%d%d",&u,&v);

		g[u].pb(v),g[v].pb(u);

	}

	REP(i,1,n) scanf("%d%d",l+i,d+i);

	ok = 1;

	vector<pii> events;

	int cnt = 0;

	multiset<int> s;

	REP(i,1,n) if (vis[i]<0) {

		A = B = {1e9,0};

		dfs(i, 0);

		if (!ok) return puts("IMPOSSIBLE"),void();

		s.insert(cur[i]=-INF);

		ID[cnt]=i,mi[cnt]=A.x,events.pb(pii(A.y,cnt)),++cnt;

		ID[cnt]=i,mi[cnt]=B.x,events.pb(pii(B.y,cnt)),++cnt;

	}

	sort(events.begin(),events.end());

	int ans = 1e9;

	for (auto &p:events) {

		s.erase(s.find(cur[ID[p.y]]));

		cur[ID[p.y]] = max(cur[ID[p.y]], mi[p.y]);

		s.insert(cur[ID[p.y]]);

		ans = min(ans, p.x-*s.begin());

	}

	printf("%d\n", ans);

}

int main() {

	int t=rd();

	REP(i,1,t) {

		printf("Case %d: ",i);

		work();

	}

}