public static int birthday(List<Integer> s, int d, int m) {
        // Write your code here
        int currSize = 0;
        int numWays = 0;
        if (s.size() < m) return 0;
        
// get sum of first segement (first m elements)
        for (int i = 0; i < m; i++) { 
            currSize += s.get(i);
        }
// check sum == d
        if (currSize == d) { 
            numWays++;
        }
        
// sliding window, subtract prev element and add next element to get sum of the next segment (size m)
// starts at 1 because you already calculated first segement sum
// ends on the last segment of size m
        for (int j = 1; j < s.size() - m + 1; j++) { 
            currSize -= s.get(j - 1);
            currSize += s.get(j + m - 1);
            
            if (currSize == d) {
                numWays++;
            }
        }
        
        return numWays;
    }