[CSES] Increasing Array

void solve() {
    ll n;
    cin >> n;

    vll a(n);
    rep(i, n) cin >> a[i];

    ll ans = 0;
    FOR(i, 1, n - 1) {
        ans += max(0, a[i - 1] - a[i]);
        a[i] = max(a[i], a[i - 1]);
    }

    cout << ans << "\n";
}