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