What am I missing here?

#1 
#include <bits/stdc++.h>

using namespace std;
typedef long long int ll;

int main()
{
string str1;
string str2;
getline(cin , str1);
getline(cin , str2);

ll n2 = str2.length();
ll n1 = str1.length();

unordered_map<char , ll> mp;
for(auto c : str2)
	mp[c]++;

ll i = 0;

while(i < n1 && mp[str1[i]] == 0)
	i++;

ll j = n1 - 1;

while(j >= 0 && mp[str1[j]] == 0)
	j--;

unordered_map<char , ll> fre;
ll sum = 0;	
for(ll k = i ; k <= j  ; k++)
{
	if(mp[str1[k]] != 0)
	{
		fre[str1[k]]++;
		sum++;
	}
}

while(i < n1 && i <= j)
{
	if(fre[str1[i]] == 0 || (fre[str1[i]] > mp[str1[i]]))
	{
		if(fre[str1[i]] != 0)
			fre[str1[i]]--;
		i++;

	}
	else
		break;
}

while(j >= 0 && i <= j)
{
	if(fre[str1[j]] == 0 || (fre[str1[j]] > mp[str1[j]]))
	{	
		if(fre[str1[j]] != 0)
			fre[str1[j]]--;			
		j--;
	}
	else
		break;
}

if(sum < n2)
	cout << "No String";
else
	cout << str1.substr(i , j - i + 1);

}