#include <iostream>
#include <vector>
#include <stack>
#include <algorithm>

using namespace std;

int trap(vector<int>& heights) {
    int n = heights.size();
    if (n <= 2) return 0; // 如果柱子少于3个，不可能存水

    int ans = 0;
    stack<int> st;

    for (int i = 0; i < n; i++) {
        while (!st.empty() && heights[i] > heights[st.top()]) {
            int prev = st.top();
            st.pop();
            if (st.empty()) break;

            int left = st.top();
            int width = i - left - 1;
            int height = min(heights[left], heights[i]) - heights[prev];
            ans += width * height;
        }
        st.push(i);
    }

    return ans;
}

int main() {
    int n;
    cin >> n;
    vector<int> heights(n);

    for (int i = 0; i < n; i++) {
        cin >> heights[i];
    }

    cout << trap(heights) << endl;

    return 0;
}