Maybe help you with my solution:

static int countingValleys(int n, string s){

    int numSeaLevel = 0;
    int numValleys = 0;

    for(int i=0;i<n;i++){
        if(s[i] == 'U')
            numSeaLevel++;

        if(s[i] == 'D') 
            numSeaLevel--;

        if(numSeaLevel==0 && s[i] == 'U'){
            numValleys++;
        }
    }
    return numValleys;
}