#include <iostream>
#include <vector>
using namespace std;

int main() {
    int n;
    cin >> n;

    vector<int> v(n);
    for(int i = 0; i < n; i++) {
        cin >> v[i];
    }
    int sum = 0;
    for(int i = 0; i < n; i++) {
        int p = v[i];
        int j = i + 1;

        while(j < n and p + 1 == v[j]) {
            j++;
            p++;
        }
        sum += v[i];
        i = j - 1;
    }
    cout << sum << endl;

    return 0;
}