#include <bits/stdc++.h>
using namespace std;
 
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vvi = vector<vector<int>>;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
#define FOR(i, m, n) for (int i = (m); i < (n); i++)
#define FORR(i, m, n) for (int i = (m); i >= (n); i--)
#define REP(i, n) FOR(i, 0, (n))
#define REPR(i, n) FORR(i, (n) - 1, 0)
#define REP1(i, n) FOR(i, 1, (n) + 1)
#define REPS(c, s) for (char c : s)
#define ALL(c) (c).begin(), (c).end()
#define SORT(c) sort(ALL(c))
#define REV(c) reverse(ALL(c))
#define sz(v) (int)v.size()
#define endl '\n'
template<class T> inline bool chmin(T& a, T b) {if (a > b) {a = b; return true;} return false;}
template<class T> inline bool chmax(T& a, T b) {if (a < b) {a = b; return true;} return false;}
template<class T> inline void printv(vector<T>& v) {int n = sz(v); REP(i, n) cout << v[i] << (i == n - 1 ? endl : ' ');}
template<class T> inline void printvv(vector<vector<T>>& v) {for (auto u : v) printv(u);}
template<class T> inline void printlnv(vector<T>& v) {int n = sz(v); REP(i, n) cout << v[i] << endl;}
const int MOD = 1000000007;
const int INF = 1000000001;
const ll LINF = 1000000001000000001LL;
 
void solve();
 
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(numeric_limits<double>::max_digits10);
    solve();
    return 0;
}

class UnionFind {
    vector<int> parent;

    public:
        UnionFind(int size) {
            parent = vector<int>(size, -1);
        }

        int find(int x) {
            if (parent[x] < 0) {
                // parent itself
                return x;
            } else {
                return parent[x] = find(parent[x]);
            }
        }

        bool unite(int x, int y) {
            int rX, rY;
            if ((rX = find(x)) == (rY = find(y))) {
                // already united
                return false;
            }
            if (size(rX) < size(rY)) {
                int tmp = rX;
                rX = rY;
                rY = tmp;
            }
            parent[rX] += parent[rY];
            parent[rY] = rX;
            return true;
        }

        int size(int x) {
            return -parent[find(x)];
        }

        bool is_same(int x, int y) {
            return find(x) == find(y);
        }

        // CAUTION!! : This method is heavy due to scanning linearly.
        int component_count() {
            int c = 0;
            for (const int x : parent) {
                if (x < 0) {
                    c++;
                }
            }
            return c;
        }
};

void solve() {
    int n;
    cin >> n;
    vi v(n + 2);
    REP1(i, n) {
        cin >> v[i];
    }
    if (n == 1) {
        cout << v[1] << endl;
        return;
    }

    int ans = 0;
    while (sz(v) > 3) {
        vi w;
        int p = -1;
        w.emplace_back(0);
        REP1(i, sz(v) - 2) {
            if (v[i] != p) w.emplace_back(v[i]);
            p = v[i];
        }
        w.emplace_back(0);
        // printv(w);
        REP1(i, sz(w) - 2) {
            if (w[i - 1] < w[i] and w[i] > w[i + 1]) {
                if (w[i - 1] > w[i + 1]) {
                    ans += w[i] - w[i - 1];
                    w[i] -= w[i] - w[i - 1];
                } else {
                    ans += w[i] - w[i + 1];
                    w[i] -= w[i] - w[i + 1];
                }
            }
        }
        v = w;
        // cout << ans << endl;
        // printv(v);
    }
    cout << ans << endl;

    
}


Measured memory_bytes: 256