My Java solution:

public static int countingValleys(int steps, String path) { int currentLevel = 0; char down = 'D'; int numberOfValleys = 0;

    for (Character c : path.toCharArray()) {
        int step = c == down ? -1 : 1;
        if (currentLevel < 0 && (currentLevel + step) == 0) {
            numberOfValleys++;
        }
        currentLevel += step;
    }
    return numberOfValleys;
}