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

const int MAX_N = 200000;
int N;
int A[MAX_N];
bool ans, tmp;

int main() {
    cin >> N;
    for (int i = 0; i < N; i++)
        cin >> A[i];
    for (int iter = 0; iter < 2; iter++) {
        tmp = true;
        for (int i = 0; i < N; i++) {
            if (A[i] > i + 1)
                tmp = false;
        }
        ans |= tmp;
        reverse(A, A + N);
    }
    cout << ((ans) ? "Yes" : "No") << endl;
    return 0;
}