#include<bits/stdc++.h>
using namespace std;
namespace ac{
	const int MAXN = 1000005, inf = 0x7fffffff / 2;
	int n, m, k, t[MAXN], u, v, d1[MAXN], top1, d2[MAXN], top2;
	int head[MAXN], cnt;
	char ch[MAXN];
	struct node{
		int next, to;
	}ed[4 * MAXN];
	void add_edge(int u, int v){
		ed[++cnt].next = head[u];
		ed[cnt].to = v;
		head[u] = cnt;
	}
	int did[MAXN], vis[MAXN];
	struct line{
		int dot, dis;
	};
	bool operator < (line a, line b){
		return a.dis > b.dis;
	}
	priority_queue<line> q;
	void dijkstra(int s){
		for(int i = 1;i <= n;i++){
			did[i] = inf;
			vis[i] = 0;
		}did[s] = t[s];
		q.push((line){s, did[s]});
		while(q.size()){
			int tmpdot = q.top().dot;
			q.pop();
			if(vis[tmpdot]) continue;
			vis[tmpdot] = 1;
			for(int i = head[tmpdot];i;i = ed[i].next){
				int v = ed[i].to;
				if(did[tmpdot] + t[v] == did[v]){
					if(tmpdot == 1) d1[++top1] = v;
					if(v == n) d2[++top2] = tmpdot;
				}
				else if(did[tmpdot] + t[v] < did[v]){
					did[v] = did[tmpdot] + t[v];
					q.push((line){v, did[v]});
					if(tmpdot == 1) d1[++top1] = v;
					if(v == n){
						top2 = 0;
						d2[++top2] = tmpdot;
					}
				}
			}
		}
	}
	int main(){
		scanf("%d%d%d", &n, &m, &k);
		for(int i = 1;i <= n;i++) scanf("%d", &t[i]);
		for(int i = 1;i <= m;i++){
			scanf("%d%d", &u, &v);
			add_edge(u, v);
			add_edge(v, u);
		}
		dijkstra(1);
		for(int i = 0;i < n;i++) ch[i] = '.';
		int n1 = k, n2 = n - k;
		if(max(n1, n2) < min(top1, top2) + 1){
			printf("impossible");
			return 0;
		} 
		if(top1 <= top2){
			if(n1 > n2){
				for(int i = 1;i <= top1;i++){
					ch[d1[i] - 1] = 'U';
					n1--;
				}ch[0] = 'U';
				n1--;
			}else{
				for(int i = 1;i <= top1;i++){
					ch[d1[i] - 1] = 'P';
					n2--;
				}ch[0] = 'P';
				n2--;
			}
		}else{
			if(n1 > n2){
				for(int i = 1;i <= top2;i++){
					ch[d2[i] - 1] = 'U';
					n1--;
				}ch[n - 1] = 'U';
				n1--;
			}else{
				for(int i = 1;i <= top2;i++){
					ch[d2[i] - 1] = 'P';
					n2--;
				}ch[n - 1] = 'P';
				n2--;
			}
		}//cout << ch << '\n';
		for(int i = 0;i < n;i++){
			if(ch[i] == '.'){
				if(n1){
					ch[i] = 'U';
					n1--;
				}else{
					ch[i] = 'P';
					n2--;
				}
			}putchar(ch[i]);
		}//printf("%d %d", n1, n2);
//		while(1);
		return 0;
	}
} int main(){
//	freopen("data.in", "r", stdin);
////	freopen("ex_data2.in", "r", stdin);
//	freopen("data.out", "w", stdout);
	return ac::main();
}//6MB