#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
int main() {
    int n;
    cin>>n;
    ll arr[n];
    for(int i=0; i<n; i++) cin>>arr[i];
    ll ans = 0;
    for(int i=1; i<n; i++) {
        int x = max(0, arr[i-1] - arr[i]);
        arr[i] += x;
        ans += x;
    }
    cout<<ans<<endl;
}