#include <iostream>

typedef unsigned int uint;

enum State {
    UNDEFINED = 0,
    DOWN = 1,
    UP = ~DOWN
};

int main() {
    uint n;
    int xPrev, xCurr;
    bool valid = true;
    int state = UNDEFINED;

    std::cin >> n;

    if(n > 0) {
        std::cin >> xCurr;
        n--;
    }

    while (n > 0 && state == UNDEFINED) {
        n--;
        xPrev = xCurr;
        std::cin >> xCurr;
        if (xPrev > xCurr) {
            state = DOWN;
        } else if (xPrev < xCurr) {
            state = UP;
        }
    }

    while(n > 0 && valid) {
        n--;
        xPrev = xCurr;
        std::cin >> xCurr;
        if(xPrev == xCurr) {
            continue;
        }

        switch(state) {
            case UP:
                valid = xPrev > xCurr;
                break;
            case DOWN:
                valid = xPrev < xCurr;
                break;
        }
        state = ~state;
    }

    std::cout << (valid ? "yes" : "no") << std::endl;

    return 0;
}