24D - Broken robot - CodeForces Solution

#include <bits/stdc++.h>
#define double long double
using namespace std;
int n, m, x, y;
double f[50005], a[1005][1005];
void gauss(int n){
	for(int i = 1; i < n; i++){
		if(i < n) a[i][i + 1] /= a[i][i];
		a[i][i - 1] /= a[i][i], a[i][n + 1] /= a[i][i], a[i][i] = 1;
		a[i + 1][n + 1] -= a[i][n + 1] * a[i + 1][i], a[i + 1][i + 1] -= a[i][i + 1] * a[i + 1][i], a[i + 1][i] = 0;
	}
	f[n] = a[n][n + 1] / a[n][n];
	for(int i = n - 1; i >= 1; i--)
		a[i][n + 1] -= a[i][i + 1] * f[i + 1], f[i] = a[i][n + 1];
}
int main(){
	cin >> n >> m >> x >> y, n = n - x + 1;
	if(m == 1) return cout << (n - 1) * 2, 0;
	for(int i = 1; i < n; i++){
		a[1][1] = 2, a[1][2] = -1, a[1][m + 1] = f[1] + 3;
		a[m][m] = 2, a[m][m - 1] = -1, a[m][m + 1] = f[m] + 3;
		for(int j = 2; j < m; j++)
			a[j][j] = 3, a[j][j - 1] = a[j][j + 1] = -1, a[j][m + 1] = f[j] + 4;
		gauss(m);
	}
	printf("%.12Lf", f[y]);
	return 0;
}