#include <cassert>

#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>

// C++
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
//#include <boost/foreach.hpp>
//#include <boost/range/algorithm.hpp>
#define rep(i,j,k) for(int i=(int)j;i<(int)k;i++)
#define ll long long
#define Sort(v) sort(all(v))
#define INF 1e9
#define LINF 1e18
#define END return 0
#define pb push_back
#define se second
#define fi first
#define pb push_back
#define all(v) (v).begin() , (v).end()
#define MP make_pair
#define MOD 1000000007LL
#define int long long
using namespace std;
int day[12]={31,28,31,30,31,30,31,31,30,31,30,31};
struct edge{int to,cost;};
typedef pair<int,int> P;

bool isupper(char c){if('A'<=c&&c<='Z')return 1;return 0;}
bool islower(char c){if('a'<=c&&c<='z')return 1;return 0;}
bool isPrime(int x){if(x==1)return 0;if(x==2)return 1;if(x%2==0)return 0;for(int i=3;i*i<=x;i++)if(x%i==0)return 0;return 1;}
bool iskaibun(string s){for(int i=0;i<s.size()/2;i++)if(s[i]!=s[s.size()-i-1])return 0;return 1;}

signed main (){
    int n;cin>>n;
    vector<int> v(n);
    rep(i,0,n)cin>>v[i];
    int ans=0;
    int l=0,r=n;
    int L=l, R=r;
    bool isL=true;
    rep(i,0,v.size()){
        if(v[i]!=0 && isL){L=i;isL=false;}
        if(v[i]!=0){R=i+1;}
    }
    l=L; r=R;
    while(1){
        bool flag=true;
        isL=true;
        int L=l, R=r;
        rep(i,0,v.size()){
            if(v[i]!=0)flag=false;
        }
        if(flag)break;
        rep(i,l,r){
            //cout<<i<<endl;
            if(v[i]==0){
                ans++;
                while(i+1<v.size()&&v[i]==v[i+1])i++;
            }
            else {
                v[i]--;
                if(v[i]!=0 && isL){L=i;isL=false;}
                if(v[i]!=0){R=i+1;}
            }
        }
        l=L; r=R;
        // rep(i,0,v.size())cout<<v[i]<<" ";
        // cout<<"lr"<<l<<" "<<r<<endl;
        ans++;
    }
    cout<<ans<<endl;
}
/*
*/


Measured memory_bytes: 256