#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  int max = a[0];
  int s = 0;
  for (int i = 1; i < n; i++) {
    if (a[i] > max)
      max = a[i];
    else if (a[i] < max) {
      for (int k = 1; k < max; k++) {
        if (a[i] + k == max) {
          s += k;
          break;
        }
      }
      a[i] = max;
    }
  }
  cout << s << endl;
}
