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