static void Main()
{
string s1 = "HUFISLKJG74KGLSKIHG7342HGJDSFG0SHFUISEHF23",
s2 = "HUFISLKJG74KGLSKIHG7342HGJDSFG1SHFUISEHF23";
Console.Write(FindDifference(s1, s2));
}
static int FindDifference(string s1, string s2)
{
for (int i = 0; i < s1.Length; i++)
{
if (s1[i] != s2[i])
return i;
}
return -1;
}