# include<iostream>

# include<cstdio>

# include<cstring>

# include<algorithm>

using namespace std;

struct Num

{

    int val,id;

};

Num num[100005],que[100005];

char p[100005];

int vis[100005];

void solve(int k,int l)

{

    int head=0,tail=-1;

    for(int i=0;i<k-1;++i){

        while(head<=tail&&que[tail].val<num[i].val)  --tail;

        ++tail;

        que[tail]=num[i];

    }

    int id=-1;

    for(int i=k-1;i<l;++i){

        while(head<=tail&&que[tail].val<num[i].val) --tail;

        ++tail;

        que[tail]=num[i];

        while(que[head].id<i-k+1||que[head].id<=id) ++head;

        printf("%d",que[head].val);

        id=que[head].id;

    }

    printf("\n");

}

int main()

{

    int n,d;

    while(scanf("%d%d",&n,&d)&&n+d)

    {

        scanf("%s",p);

        int l=strlen(p);

        for(int i=0;i<l;++i)

            num[i].val=p[i]-'0',num[i].id=i;

        solve(d+1,l);

    }

    return 0;

}