#include<queue>
#include<cstdio>
#include<iostream>

using namespace std;

typedef long long LL;

priority_queue<int,vector<int>,greater<int> >q;

void read(int &x)
{
    x=0; char c=getchar();
    while(!isdigit(c)) c=getchar();
    while(isdigit(c)) { x=x*10
 
+c-'0'; c=getchar(); }
}

int main()
{
    freopen("stock.in","r",stdin);
    freopen("stock.out","w",stdout);
    int n,x,y;
    LL ans=0;
    read(n);
    read(x);
    ans-=x;
    for(int i=2;i<=n;++i)
    {
        read(x);
        if(!(i&1)) 
        {
            ans+=x;
            q.push(x);
        }
        
 
else
        {
            y=q.top();
            if(x>y)
            {
                ans+=x-y*2;
                q.pop();
                q.push(x);
            }
            else ans-=x;
        }
    }
    cout<<ans;
}