#include <iostream>
#include <fstream>
using namespace std;
ifstream in("operatii.in");
ofstream out("operatii.out");
const int maxn = 1000005;
int v[maxn];
int main()
{
    int n;
    in >> n;
    for(int i = 1; i <= n; i++)
        in >> v[i];
    int incep = 1;
    int nrmut = 0;
    while(incep <= n)
    {
        while(incep < n && v[incep + 1] == 0)
            incep++;
        int fin = incep;
        int last = v[incep];
        while(fin < n && v[fin + 1] >= last)
        {
            fin++;
            last = v[fin];
            v[fin] -= v[incep];
        }
        nrmut = nrmut + v[incep];
        v[incep] = v[incep] - fin;
        incep++;
    }
    out << nrmut << "\n";
    return 0;
}