#include<iostream>
#include<stdlib.h>
#include<algorithm>
#include<cmath>
using namespace std;

long toDecimal(string str,int radix){
    reverse(str.begin(),str.end());
    long res=0;
    for(unsigned int i=0;i<str.length();i++){
        if(str[i]>='0'&&str[i]<='9'){
            res+=(str[i]-'0')*pow(radix,i);
        }else{
            res+=(str[i]-'a'+10)*pow(radix,i);
        }
    }
    return res;
}

int main(){
    string N1,N2;
    int tag,radix;
    cin>>N1>>N2>>tag>>radix;
    long res1,res2;
    string a;
    if(tag==1){
        res1=toDecimal(N1,radix);
        a=N2;
    }else{
        res1=toDecimal(N2,radix);
        a=N1;
    }
    int flag=0,max=0;
    for(unsigned int i=0;i<a.length();i++){
        int temp;
        if(a[i]>='0'&&a[i]<='9'){
            temp=a[i]-'0';
        }else{
            temp=a[i]-'a'+10;
        }
        if(temp>max){
            max=temp;
        }
    }
    long left=max+1,right=res1+1;
    while(left<=right){
        long mid=(left+right)/2;
        res2=toDecimal(a,mid);
        if(res2==res1){
            cout<<mid;
            flag=1;
            break;
        }else if(res2>res1||res2<0){
            right=mid-1;
        }else{
            left=mid+1;
        }
    }
    if(flag==0){
        cout<<"Impossible";
    }
    return 0;
}