#include <cstdio>#include <cstring>#include <cmath>#include <algorithm>#include <climits>#include <string>#include <iostream>#include <map>#include <cstdlib>#include <list>#include <set>#include <queue>#include <stack>using namespace std;typedef long long LL;const int maxn = 800 + 5;const int maxm = 80000;const int INF = INT_MAX / 2;int n,m,s,t;int first[maxn],nxt[maxm];int ecnt,v[maxm],cap[maxm];int q[maxn],qs,qe;bool vis[maxn];void adde(int uu,int vv,int ww) {    v[ecnt] = vv; cap[ecnt] = ww; nxt[ecnt] = first[uu]; first[uu] = ecnt++;    v[ecnt] = uu; cap[ecnt] = 0; nxt[ecnt] = first[vv]; first[vv] = ecnt++;}int level[maxn];bool bfs() {    memset(level,0,sizeof(level));    qs = qe = 0;    q[qe++] = s;    level[s] = 1;    while(qs < qe) {        int now = q[qs++];        if(now == t) break;        for(int i = first[now];~i;i = nxt[i]) {            if(cap[i] && level[v[i]] == 0) {                q[qe++] = v[i]; level[v[i]] = level[now] + 1;            }        }    }    return level[t];}int dfs(int now,int alpha) {    if(now == t) return alpha;    int sum = 0;    for(int i = first[now];~i && alpha;i = nxt[i]) {        if(cap[i] && level[v[i]] == level[now] + 1) {            int ret = dfs(v[i],min(alpha,cap[i]));            sum += ret; alpha -= ret;            cap[i] -= ret; cap[i ^ 1] += ret;        }    }    if(sum == 0) level[now] = -1;    return sum;}void solve() {    //先做一遍最大流       while(bfs()) dfs(s,INF);    //分别从起点和终点做一遍bfs    memset(vis,0,sizeof(vis));    qs = qe = 0;    q[qe++] = s;    vis[s] = true;    while(qs < qe) {        int now = q[qs++];        for(int i = first[now];~i;i = nxt[i]) {            if(cap[i] && !vis[v[i]]) {                vis[v[i]] = true; q[qe++] = v[i];            }        }    }    qs = qe = 0;    q[qe++] = t;    vis[t] = true;    while(qs < qe) {        int now = q[qs++];        for(int i = first[now];~i;i = nxt[i]) {            if(cap[i ^ 1] && !vis[v[i]]) {                vis[v[i]] = true; q[qe++] = v[i];            }        }    }    for(int i = 1;i <= n;i++) {        if(!vis[i]) {            puts("AMBIGUOUS");            return;        }    }    puts("UNIQUE");}int main() {    while(scanf("%d%d%d%d",&n,&m,&s,&t),n) {        memset(first,-1,sizeof(first));        ecnt = 0;        for(int i = 0;i < m;i++) {            int a,b,c; scanf("%d%d%d",&a,&b,&c);            adde(a,b,c); adde(b,a,c);        }        solve();    }    return 0;}