Operations on array

```cpp
#include <iostream>
#include <vector>
using namespace std;
int main(){
  int n;
  vector<int> arr(n);
  for(int i = 0; i<n; i++) cin >> arr[i];
  ll ans = 0;
  for(int i = 1; i<n; i++){
    if(a[i] < a[i-1]){
      ans += a[i-1] - a[i];
    }
  }
  cout << ans << "\n";
  return 0;
}
```