用分治归并排序都超时？？？求大哥解释一下

#include<bits/stdc++.h>
using namespace std;
const int N = 5000005;
int is1[N],temp[N];
void merge(int a,int mid,int b){
	int i = a,j = mid + 1,k = a;
	while(i<=mid&&j<=b){
		if(is1[i]<is1[j]) temp[k++] = is1[i++];
		else temp[k++] = is1[j++];
	}
	while(i<=mid) temp[k++] = is1[i++];
	while(j<=b) temp[k++] = is1[j++];
    for(int i = a;i<=b;i++){
        is1[i]=temp[i];
    }
}
void mergesort(int a,int b){
	if(a>=b) return;
	int mid = (a+b)/2;
	mergesort(a,mid);
	mergesort(mid+1,b);
	merge(a,mid,b);
}
int main(){
int n,m;
scanf("%d  %d ",&n,&m);
for(int i=1;i<=n;i++){
    scanf("%d",&is1[i]);
}
mergesort(1,n);
printf("%d",is1[m+1]);
return 0;
}