#include <bits/stdc++.h>
#define int long long
using namespace std;
signed main(){
    int n,m;
    cin>>n>>m;
    string a,b;
    cin>>a>>b;
    vector<int> hashA,hashB;
    for(int i=0;i<n;++i){
        if(a[i]=='#') hashA.push_back(i); 
        if(b[i]=='#') hashB.push_back(i); 
    }
    for(int i=0;i<m;++i){
        char replaceChar='a'+(i%26); 
        if(!hashA.empty()){
            int pos=hashA.front();
            a[pos]=replaceChar; 
            hashA.erase(hashA.begin()); 
        }else if(!hashB.empty()){ 
            int pos=hashB.front();
            b[pos]=replaceChar; 
            hashB.erase(hashB.begin()); 
        }
    }
    cout<<a<<endl; 
    return 0;
}