BAX

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

typedef long long ll;
#define pii pair<int , int>
#define _FastIO ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define MAXX 200005

ll max(ll a , ll b){
    if(a > b)   return a;
    else    return b;
}

ll a[MAXX];
ll n , cvb;

int main()
{
    _FastIO;
    cin >> n;
    for(ll i = 0; i < n; i++){
        cin >> a[i];
    }
    for(ll i = 1; i < n; i++){
        cvb += max(0 , a[i - 1] - a[i]);
        a[i] = max(a[i] , a[i - 1]);
    }
    cout << cvb << endl;
    return 0;
}