CF24D Broken robot

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#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;
#define re register
const int MAXn = 1e3;
const int MAXm = 1e3;

template <class T> 
inline void read(T &a) {
    register char c;while (c = getchar(), c < '0' || c > '9');register T x(c - '0');while (c = getchar(), c >= '0' && c <= '9') {x = (x << 1) + (x << 3) + (c ^ 48);}a = x;  
}

int n, m, x, y;
double mat[MAXm + 10][MAXm + 10], d[MAXn + 10][MAXm + 10];
signed main() {
    read(n), read(m);
    read(x), read(y);
    if (m == 1) {
        printf("%d\n", 2 * (n - x));
        return 0;	
    }
    for (re int i = n - 1; i >= x; --i) {
        memset(mat, 0, sizeof(mat));
        mat[1][1] = 2; mat[1][2] = -1;
        mat[1][m + 1] = d[i + 1][1] + 3;
        for (re int j = 2; j < m; ++j) {
            mat[j][j - 1] = mat[j][j + 1] = -1; mat[j][j] = 3;
            mat[j][m + 1] = d[i + 1][j] + 4;
        }
        mat[m][m] = 2; mat[m][m - 1] = -1;
        mat[m][m + 1] = d[i + 1][m] + 3;
        // ------- 高斯消元 begin -------
        for (re int i = 1; i < m; ++i) {
            double solve = mat[i + 1][i] / mat[i][i];
            mat[i + 1][i] -= mat[i][i] * solve;
            if (i != m) {
                mat[i + 1][i + 1] -= mat[i][i + 1] * solve;
            }
            mat[i + 1][m + 1] -= mat[i][m + 1] * solve;
        }
        for (re int i = m; i > 1; --i) {
            double solve = mat[i - 1][i] / mat[i][i];
            mat[i - 1][i] -= mat[i][i] * solve;
            mat[i - 1][m + 1] -= mat[i][m + 1] * solve;
        }
        // ------- 高斯消元 end -------
        for (re int j = 1; j <= m; ++j) {
            d[i][j] = mat[j][m + 1] / mat[j][j];
        }
    }
    printf("%.4f\n", d[x][y]);
}