#include<bits/stdc++.h>
using namespace std;
const int N=1e6+5;;
char a[N],b[N];
int main()
{
	int n;
	cin>>n;
	cin>>a;
	cin>>b;
	int ans=0;
	int i,j=n-1;
	for(i=n-1;i>=0;i--){
		if(a[i]==b[j]) j--;
		else ans++;
	}
	cout<<ans;
	return 0;
}