HDU 2604 Queuing

#include<cstdio>
#include<cstring>
using namespace std;
int m;
int f(int x)
{
    int s1[5][5],s2[5][5],s3[5][5],i,a=0,j,k,b=0,c=0,d=0;
    memset(s1,0,sizeof(s1));
    memset(s2,0,sizeof(s2));
    s1[0][1]=1;s2[0][1]=1;
    s1[1][3]=1;s2[1][3]=1;
    s1[2][0]=1;s2[2][0]=1;
    s1[2][1]=1;s2[2][1]=1;
    s1[3][2]=1;s2[3][2]=1;
    s1[3][3]=1;s2[3][3]=1;
   while (x)
   {
          if (x&1)
          {
             memset(s3,0,sizeof(s3));
             for (i=0;i<4;i++)
             for (j=0;j<4;j++)
             for (k=0;k<4;k++)
             s3[i][j]+=s1[i][k]*s2[k][j];
             for (i=0;i<4;i++)
             for (j=0;j<4;j++)
               s2[i][j]=s3[i][j]%m;
        }
        memset(s3,0,sizeof(s3));
        for (i=0;i<4;i++)
        for (j=0;j<4;j++)
        for (k=0;k<4;k++)
        s3[i][j]+=s1[i][k]*s1[k][j];
        for (i=0;i<4;i++)
        for (j=0;j<4;j++)
        s1[i][j]=s3[i][j]%m;
        x>>=1;
    }
    for (i=0;i<4;i++)
    {
        a+=s2[i][0];
        b+=s2[i][1];
        c+=s2[i][2];
        d+=s2[i][3];
    }
    return a+b+c+d;
}
int main()
{
    int l,p;
    while (~scanf("%d%d",&l,&m))
    {
        if (l==0) {printf("0\n");continue;}
        if (l==1) {printf("%d\n",2%m);continue;}
        if (l==2) {printf("%d\n",4%m);continue;}
        l-=3;
        p=f(l)%m;
        printf("%d\n",p);
    }
    return 0;
}