#include<bits/stdc++.h>

using namespace std;

int main(){
    int i,j,k,l;
    int m,n;
    int h,x,y;
    
    cin>>n>>m;
    
    vector<int> v;
    
    l=0;
    for (i=0;i<m;i++){
        cin>>j;
        v.push_back(j);
    }
    sort(v.begin(),v.end(),[](int l,int r){return abs(l)<abs(r);});
    
    if (n==1){
        cout<<abs(v[0])<<endl;
        return 0;
    }
    
    l=h=v[0];
    for (i=1;i<n;i++){
        if (v[i]<l)l=v[i];
        if (h<v[i])h=v[i];
    }
    if (0<=l){
        cout<<h<<endl;
    }else if(0<h){
        cout<<(abs(l)*2+h)<<endl;
    }else {
        cout<<(abs(l))<<endl;
    }
    
    return 0;
}