#include <cstdio>

 #include <iostream>

 using namespace std;

 const int N = ;

 int n, k;

 char s[N], stk[N], mx[N];

 int main() {

     while (~scanf("%d%d%s", &n, &k, s)) {

         int top = -;

         if (k == n - ) {

             char mn = '';

             for (int i = ; i < n; i++) mn = min(s[i], mn);

             putchar(mn); puts("");

             continue;

         }

         for (int i = ; i < n; i++) {

             if (s[i] != '') while (~top && k >= top +  && mx[top] > s[i]) top--, k--;

             while (k && ~top && stk[top] > s[i]) {

                 if (s[i] == '' && top == ) break;

                 top--; k--;

             }

             stk[++top] = s[i];

             if (top > ) mx[top] = max(s[i], mx[top - ]);

             else mx[top] = s[i];

         }

         while (k--) top--;

         //printf("top %d\n", top);

         for (int i = ; i <= top; i++) putchar(stk[i]); puts("");

     }

     return ;

 }