#include <iostream>
#include <vector>

using namespace std;

int main() {
    int N;
    cin >> N;
    vector<int> H;
    for (int i = 0; i < N; ++i) {
        int tmp;
        cin >> tmp;
        H.push_back(tmp);
    }
    vector<int> left(N);
    left[0] = H[0];
    for (int i = 1; i < N; ++i) {
        left[i] = max(H[i], left[i-1]);
    }

    vector<int> right(N);
    right[N-1] = H[N-1];
    for (int i = N-2; i >= 0; --i) {
        right[i] = max(H[i], right[i+1]);
    }

    int ans = 0;
    for (int i = 0; i < N; ++i) {
        ans += min(left[i], right[i]) - H[i];
    }

    cout << ans;

    return 0;
}