Longest subsequence

String is a very useful thing and a subsequence of the same string is equally important.

Now you have a string ss with length nn and a string tt with length mm. Find out the longest subsequence in the string ss so that the lexicographical order of this subsequence is strictly larger than tt.

Input

two integers nn, mm in the first line 

（All characters are lowercase letters）

The second line is a string ss

The third line is a string tt

• 1 \le n,m \le 10^61≤n,m≤106

Output

Output an integer representing the longest length, otherwise output -1.

样例输入1复制

9 3
aaabbbccc
abc

样例输出1复制

6

样例输入2复制

9 3
aaabbbccc
zzz

样例输出2复制

-1

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn=1e6+10;

int n,m;
char s[maxn],t[maxn];
int nx[maxn][26];

int main() {
    scanf("%d%d",&n,&m);
    scanf("%s%s",s+1,t+1);
    for(register int i=0;i<26;++i){
        nx[n][i]=nx[n+1][i]=n+1;
    }
    for(register int i=n-1;i>=0;--i){
        for(register int j=0;j<26;++j){
            nx[i][j]=nx[i+1][j];
        }
        int to=s[i+1]-'a';
        nx[i][to]=i+1;
    }
    int res=-1,cur=0;
    for(register int i=1;t[i];++i){
        int pos=t[i]-'a';
        for(register int j=pos+1;j<26;++j){
            int id=nx[cur][j];
            if(id<=n){
                res=max(res,n-id+1+i-1);
            }
        }
        cur=nx[cur][pos];
        if(cur>n)break;
    }
    if(cur<n)res=max(res,m+n-cur);
    printf("%d\n",res);
    return 0;
}