AC

#include <bits/stdc++.h>

using namespace std;

//统计字符串A中子串出现次数
int countOccurrences(const string &A, const string &pattern)
{
    int count = 0;
    int pos = 0;
    while ((pos = A.find(pattern, pos)) != string::npos) 
    {
        count++;
        pos++; 
    }
    return count;
}

string A,B;
int n;
int main() 
{
    cin >> A >> B >> n;
    int maxLen = 0; 
    int low = 1, high = B.size();
    //立即启动二分查找！
    while (low <= high)
    {
        int mid = low + (high - low) / 2;
        string prefix = B.substr(0, mid); 
        int occurrences = countOccurrences(A, prefix); 

        if (occurrences >= n)
        {
            maxLen = mid; 
            low = mid + 1; 
        } 
        else
            high = mid - 1; 
 }

    if (maxLen == 0) 
        cout << "IMPOSSIBLE" << endl;
    else 
        cout << B.substr(0, maxLen) << endl; 
    return 0;
}