Given two strings. The task is to find the length of the longest common substring.

Example 1:

Input: S1 = "ABCDGH", S2 = "ACDGHR", n = 6, m = 6
Output: 4
Explanation: The longest common substring
is "CDGH" which has length 4.

Example 2:

Input: S1 = "ABC", S2 "ACB", n = 3, m = 3
Output: 1
Explanation: The longest common substrings
are "A", "B", "C" all having length 1.

Your Task:
You don't need to read input or print anything. Your task is to complete the function longestCommonSubstr() which takes the string S1, string S2 and their length n and m as inputs and returns the length of the longest common substring in S1 and S2.

Expected Time Complexity: O(n*m).
Expected Auxiliary Space: O(n*m).

Constraints:
1<=n, m<=1000