#include <bits/stdc++.h>
using namespace std;
int n;
long long a[200005], ans;
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) {
    scanf("%lld", &a[i]);
  }
  for (int i = 2; i <= n; i++) {
    if (a[i] < a[i - 1]) {
      a[i] += a[i] - a[i - 1];
      ans += abs(a[i] - a[i - 1]);
    }
  }
  printf("%lld\n", ans);
  return 0;
}
