CF24D Broken robot

#include<bits/stdc++.h>
using namespace std;
const int N=1007;
int n,m,x,y;
double f[N],a[N][N],b[N];
void build(){
    a[1][1]=2.0/3.0;
    a[m][m]=2.0/3.0;
    a[1][2]=-1.0/3.0;
    a[m][m-1]=-1.0/3.0;
    for(int i=2;i<=m-1;i++){
        a[i][i]=3.0/4.0;
        a[i][i-1]=-1.0/4.0;
        a[i][i+1]=-1.0/4.0;
    }
}
void gauss(){
    for(int i=1;i<=m;i++){
        double rate=a[i+1][i]/a[i][i];
        a[i+1][i]-=rate*a[i][i];
        a[i+1][i+1]-=rate*a[i][i+1];
        b[i+1]-=rate*b[i];
    }
    f[m]=b[m]/a[m][m];  
    for(int i=m-1;i>0;i--){
        f[i]=(b[i]-f[i+1]*a[i][i+1])/a[i][i];
    }
}
int main(){
    cin>>n>>m>>x>>y;
    if(m==1){
        printf("%.8lf\n",(double)2.0*(n-x));
        return 0;
    }
    for(int i=n-1;i>=x;i--){
        b[1]=f[1]/3.0+1.0;
        b[m]=f[m]/3.0+1.0;
        for(int j=2;j<m;j++){
            b[j]=f[j]/4+1.0;
        }
        build();
        gauss();
    }
    printf("%.8lf\n",f[y]);
    return 0;
}