#include <iostream>
#include <string>
using namespace std;

int main() {
	int m, n, s, k;
	cin >> m >> n >> s >> k;
	string str;
	cin >> str;
	int star_number = 0;
	bool end_of_stars = 0;
	bool sec_seq = 0;
	bool imp = 0;
	bool un = 0;
	bool amb = 0;
	for (int i = 0; i < str.length() && !sec_seq; i++){
		if (str[i] == '*') {
			if (end_of_stars == 1) sec_seq = 1;
			else star_number++;
		}
		else {
			if ( (star_number != 0) and (end_of_stars == 0) ) end_of_stars = 1;
		}
	}
	if (sec_seq == 1) imp = 1;
	else {
		if (star_number == 0) {
			if (s > k-1 && s > m-k) imp = 1;
			else {
				if (n>s) amb = 1;
				else {
					if ( ( (s > m - k) and (k - 1 == s) ) or ( (s > k - 1) and (m - k == s) ) ) un = 1;
					else amb = 1;
				}
			}
		}
		else {
			if (star_number != s) imp = 1;
			else {
				if (m == s || s == 1 || k == 1 || k == m) un = 1;
				else amb = 1;
			}
		}
	}
	if (un == 1) cout << "Unique";
	if (amb == 1) cout << "Ambiguous";
	if (imp == 1) cout << "Impossible";
	return 0;
}