#include <iostream>
#include <cctype>
#include <algorithm>
#include <cmath>
using namespace std;
long long convert(string n, long long radix) {
long long sum = 0;
int index = 0, temp = 0;
for (auto it = n.rbegin(); it != n.rend(); it++) {
temp = isdigit(*it) ? *it - '0' : *it - 'a' + 10;// - 'a' + 10 transform letter to 10radix number;-'0' transform string to number;(
a-z represent the decimal numbers 10-35.) sum += temp * pow(radix, index++);
}
return sum;
}
long long find_radix(string n, long long num) {
char it = *max_element(n.begin(), n.end());
long long low = (isdigit(it) ? it - '0' : it - 'a' + 10) + 1;
long long high = max(num, low);
while (low <= high) {
long long mid = (low + high) / 2;
long long t = convert(n, mid);
if (t < 0 || t > num) //if t is negative number,that mean t out of long long boundary.
high = mid - 1;
else if (t == num) return mid;
else low = mid + 1;
}
return -1;
}
int main() {
string n1, n2;
long long tag = 0, radix = 0, result_radix;
cin >> n1 >> n2 >> tag >> radix;
result_radix = tag == 1 ? find_radix(n2, convert(n1, radix)) : find_radix(n1, convert(n2, radix));
if (result_radix != -1) {
printf("%lld", result_radix);
}
else {
printf("Impossible");
}
return 0;
}