#include <iostream.h>
void main() {
	int n,i=0,s=0,j=1,a,b=0,c;
	cin>>n;
	while(i<n){
		if(j%2) {
			cin>>a,i++;
			while(a>=b) {
				b=a;
				if(i==n) break;
				cin>>a,i++;
			} s+=b;j++;
		}
		else {
			cin>>c,i++;
			while(c<=a) {
				a=c;
				if(i==n) break;
				cin>>c,i++;
			} 
			if(i!=n) s-=a,j++;
			b=c;
		}
	}
	cout<<s<<endl;
}