keoki

#include <stdio.h>
#include <stdlib.h>



#ifndef DEBUG
#define DEBUG(...) printf(__VA_ARGS__)
#endif

#define MAXN 100001
int subsum(int *a, int n) {
  if(*a == a[n-1]) return 0;
  return *a + subsum(++a, n)*(-1);
}


int main() {
  int n, a[MAXN];
  scanf("%d", &n);
  for (int i = 0; i < n; ++i) {
    scanf("%d", &a[i]);
  }
  printf("%d\n", subsum(a, n));
  return 0;
}