int kmp(string a1,string a2)
{
	int n=a1.size(),m=a2.size();
	//cout<<"n="<<n<<" m="<<m<<endl;
	string s=a1 + '#' + a2;
	//cout<<s<<endl;       
	vector<int> pi(s.size());
	for (int i=n;i<s.size();i++)
	{
		int len=pi[i-1];
		while(len != 0 && s[i] != s[len]){
			len=pi[len-1];
		}
		//cout<<"len="<<len<<endl;
		//cout<<"pi["<<i<<"]="<<pi[i]<<endl;
		if(s[i] == s[len]){
			pi[i] = ++len;
			//cout<<"pi["<<i<<"]="<<pi[i]<<endl;
			if(pi[i] == n) return i - n * 2;
		}
	}
	return -1;
}