#define _CRT_SECURE_NO_DEPRECATE
#include<iostream>
#include<algorithm>
#include<cmath>
#include<cstring>
#include<string>
#include<set>
#include<queue>
using namespace std;
#define INF 0x3f3f3f
#define N_MAX 30+5
#define M_MAX 2001
typedef unsigned long long ll;
int tag,radix;
struct Number {string number;ll base;
}num[2];
bool known , unknown;
ll sum1, sum2;
ll translate_to_10(string s,ll base) {//转化成十进制ll sum = 0;for (int i = 0; i < s.size();i++) {if (s[i] >= '0'&&s[i] <= '9') {sum = sum*base + s[i] - '0';}else if (s[i] >= 'a'&&s[i] <= 'z') {sum = sum*base + s[i] - 'a' + 10;}}return sum;
}bool C(ll x) {ll sum = translate_to_10(num[unknown].number, x);if (sum<0)return true;//太大导致数值溢出if (sum >= sum1)return true;else return false;
}int main() {while (cin>>num[0].number>>num[1].number>>tag>>radix) {known = !(tag & 1),unknown=!known;num[known].base = radix;sum1 = translate_to_10(num[known].number, num[known].base);int strart_base = 2; string s = num[unknown].number;for (int i = 0; i < s.size(); i++) {//确定未知数至少是多少进制if (s[i] >= '2'&&s[i] <= '9'&&strart_base < s[i] - '0'+1)strart_base = s[i] - '0'+1;else if (s[i] >= 'a'&&s[i] <= 'z'&&strart_base < s[i] - 'a' + 10+1)strart_base = s[i] - 'a' + 10+1;}ll lb = strart_base-1, ub = sum1 + 1;if (translate_to_10(num[unknown].number, lb) > sum1) { puts("Impossible"); continue; }//剪枝while (ub-lb>1) {ll mid = (lb + ub) >> 1;if (C(mid))ub = mid;else lb = mid;}if (translate_to_10(num[unknown].number, ub) == sum1){printf("%lld\n",ub);}else puts("Impossible");}return 0;
}