#include<bits/stdc++.h>
using namespace std;
int n,m,cc,aa;
string a,b;
queue<int>a1;
queue<int>a2;
int main(){
    cin>>n>>m;
    cin>>a>>b;
    for(int i=0;i<n;i++){
        if(a[i]=='#'){
        	a1.push(i);
		}
        if(b[i]=='#'){
        	a2.push(i);
		}
    }
    for(int i=0;i<m;i++){
        cc=(i%26)+1;
        if(!a1.empty()){
            aa=a1.front();
            a[aa]='a'+cc-1;
            a1.pop();
        }
		else if(!a2.empty()){
            aa=a2.front();
            b[aa]='a'+cc-1;
            a2.pop();
        }
    }
    cout<<a;
    return 0;
}