// compile: g++ -o data data.cpp -std=gnu++20 -DLOCAL
// run: ./data < data.in
#include <bits/stdc++.h>
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#ifdef LOCAL
#endif
#define sz(v) ((int)(v).size())
#define PV(v) for (int i=0;i<v.size();++i)cout<<v[i]<<" ";cout<<endl;
#define INTINF 9223372036854775807
#define int long long
 
using namespace std;
 
int32_t main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    int n; cin >> n;
    vector<int> a(n);
    for (int i = 0; i < n; ++i) cin >> a[i];
    int ans = 0;
    for (int i = 1; i < n; ++i) {
        if (a[i] >= a[i-1]) continue;
        ans += a[i-1] - a[i];
        a[i] = a[i-1];
    }
    cout << ans << endl;
 
    return 0;
}