T1

#include <iostream>
#include <vector>

using namespace std;

int main() {
    int n;
    cin >> n;
    vector<long long> a(n);  
    
    for (int i = 0; i < n; ++i) {
        cin >> a[i];
    }
    
    long long operations = 0;  
    
    for (int i = 1; i < n; ++i) {
        if (a[i] < a[i-1]) {
            operations += a[i-1] - a[i];
            a[i] = a[i-1];  
        }
    }
    
    cout << operations << endl;
    
    return 0;
}