#include <iostream>
#include <vector>
#include <string>
#include <iomanip>
#include <queue>
#include <cmath>
#include <algorithm>
#include <tuple>
#include <iomanip>
#include <stdio.h>
#include <map>
#include <bitset>
#include <set>
#include <stack>
#include <queue>
#include <unordered_set>
#include <cassert>


//#pragma GCC optimize("Ofast,no-stack-protector")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
//#pragma GCC optimize("unroll-loops")
//#pragma GCC optimize("fast-math")
//#pragma GCC optimize("section-anchors")
//#pragma GCC optimize("profile-values,profile-reorder-functions,tracer")
//#pragma GCC optimize("vpt")
//#pragma GCC optimize("rename-registers")
//#pragma GCC optimize("move-loop-invariants")
//#pragma GCC optimize("unswitch-loops")
//#pragma GCC optimize("function-sections")
//#pragma GCC optimize("data-sections")
//#pragma GCC optimize("branch-target-load-optimize")
//#pragma GCC optimize("branch-target-load-optimize2")
//#pragma GCC optimize("btr-bb-exclusive")


#define int long long
#define ll long long
#define ull unsigned long long
#define all(a) a.begin(), a.end()
#define pii pair<int, int>
#define pb push_back
#define ld long double


using namespace std;

const int INF = 1e17;
//const int mod = 2600000069;
//const int p = 179;

signed main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    string a, b, c;
    cin >> a >> b >> c;
    string aa, bb, cc;
    aa = a[0];
    bb = b[0];
    cc = c[0];
    int s1 = a.size(), s2 = b.size(), s3 = c.size();
    for (int i = 1; i < s1; i++) {
        if (a[i] != a[i - 1]) {
            aa += a[i];
        }
    }
    for (int i = 1; i < s2; i++) {
        if (b[i] != b[i - 1]) {
            bb += b[i];
        }
    }
    for (int i = 1; i < s3; i++) {
        if (c[i] != c[i - 1]) {
            cc += c[i];
        }
    }
    if (aa != bb || bb != cc) {
        cout << "IMPOSSIBLE";
        return 0;
    }
    int k = aa.size();
    vector<int> num1, num2, num3;
    int cnt = 1;
    char cur = a[0];
    for (int i = 1; i < s1; i++) {
        if (a[i] != cur) {
            num1.pb(cnt);
            cur = a[i];
            cnt = 1;
        } else {
            cnt++;
        }
    }
    num1.pb(cnt);
    cur = b[0];
    cnt = 1;
    for (int i = 1; i < s2; i++) {
        if (b[i] != cur) {
            num2.pb(cnt);
            cur = b[i];
            cnt = 1;
        } else {
            cnt++;
        }
    }
    num2.pb(cnt);
    cur = c[0];
    cnt = 1;
    for (int i = 1; i < s3; i++) {
        if (c[i] != cur) {
            num3.pb(cnt);
            cur = c[i];
            cnt = 1;
        } else {
            cnt++;
        }
    }
    num3.pb(cnt);
    string ans = "";
    for (int i = 0; i < k; i++) {
        vector<int> qqq = {num1[i], num2[i], num3[i]};
        sort(all(qqq));
        for (int j = 0; j < qqq[1]; j++) {
            ans += aa[i];
        }
    }
    cout << ans;
}
/*

*/