#include <bits/stdc++.h>
using namespace std;

int main() {
    int n;
    cin>>n;
    vector<long long> a(n);
    for(int i=0;i<n;i++) cin>>a.at(i);

    long long sum=0;
    for(int i=1;i<n;i++){
        long long diff=a.at(i-1)-a.at(i);
        sum+=max(0,diff);
        a.at(i)+=max(0,diff);
    }
    cout<<sum<<endl;
}