#include <cstdlib>
#include <cstring>
#include <cstdio>
#include <iostream>
#include <algorithm>
using namespace std;const int SS = 805, TT = 806;
const int INF = 0x3fffffff;
int N, M, A, B;struct Edge {int v, c, next;
};
Edge e[30000];
int idx, head[810], lv[810];
int front, tail, que[810];
int vis[810];void insert(int a, int b, int c) {e[idx].v = b, e[idx].c = c;e[idx].next = head[a];head[a] = idx++;
}bool bfs() {memset(lv, 0xff, sizeof (lv));front = tail = lv[SS] = 0;que[tail++] = SS;while (front < tail) {int u = que[front++];for (int i = head[u]; i != -1; i = e[i].next) {int v = e[i].v;if (!(~lv[v]) && e[i].c) {lv[v] = lv[u] + 1;if (v == TT) return true;que[tail++] = v;    }}}return false;
}int dfs(int u, int sup) {if (u == TT) return sup;int tf = 0, f;for (int i = head[u]; i != -1; i = e[i].next) {int v = e[i].v;if (lv[u]+1==lv[v] && e[i].c && (f=dfs(v, min(e[i].c, sup-tf)))) {tf += f;e[i].c -= f, e[i^1].c += f;if (tf == sup) return sup;    }}if (!tf) lv[u] = -1;return tf;
}void dinic() {int ret = 0;while (bfs()) {ret += dfs(SS, INF);    }
//    printf("ret = %d\n", ret);
}void flood(int u, int &cnt) {for (int i = head[u]; i != -1; i = e[i].next) {if (e[i].c && !vis[e[i].v])    {++cnt;vis[e[i].v] = 1;flood(e[i].v, cnt);}}
}void flood_r(int u, int &cnt) {for (int i = head[u]; i != -1; i = e[i].next) {if (e[i^1].c && !vis[e[i].v])    {++cnt;vis[e[i].v] = 1;flood_r(e[i].v, cnt);}}
}bool query() {int cnta = 0, cntb = 0; // 分别为从两个方向进行搜索而计数memset(vis, 0, sizeof (vis));vis[SS] = vis[TT] = 1;flood(SS, cnta);memset(vis, 0, sizeof (vis));vis[SS] = vis[TT] = 1;flood_r(TT, cntb);return cnta + cntb == N;
}int main() {while (scanf("%d %d %d %d", &N, &M, &A, &B), N|M|A|B) {int a, b, c;idx = 0;memset(head, 0xff, sizeof (head));insert(SS, A, INF), insert(A, SS, 0);insert(B, TT, INF), insert(TT, B, 0);for (int i = 0; i < M; ++i) {scanf("%d %d %d", &a, &b, &c);insert(a, b, c), insert(b, a, c);}dinic();printf(query() ? "UNIQUE\n" : "AMBIGUOUS\n");}return 0;
}