C++ :

#include<iostream>
#include<string>
#include<algorithm>
#include<cstring>
using namespace std;
const int N(55);
int f[N][N]={0};

int main()
{
    string s1,s2;
    int i,j,len1,len2;
    while (cin>>s1>>s2)
    {    
        memset(f,0,sizeof(f));       
        len1=s1.size();  len2=s2.size();
        for (i=1;i<=len1;i++)
            for (int  j=1;j<=len2;j++)
            {                 
                f[i][j]=f[i-1][j-1]+(s1[i-1]==s2[j-1]?2:0); 
                for (int k=1;k<i;k++) f[i][j]=max(f[i][j],f[k][j]-1);
                for (int k=1;k<j;k++) f[i][j]=max(f[i][j],f[i][k]-1);
            }
        cout<<f[len1][len2]<<endl;
    }
    return 0;
}