题目理解问题，关于谷底s[i][j]的值

按照题解的思路，谷底对应的值是1。 为什么谷底对应的值不是0，也就是滑不了呢？

#include<iostream>
#include<vector>
using namespace std;
/*
例如测试用例
4 4
10 9 8 7
10 5 8 8
10 9 9 9
10 9 9 9
输出
0 0 0 0 0 0
0 4 3 2 1 0
0 1 0 1 2 0
0 2 1 2 3 0
0 2 1 1 1 0
0 0 0 0 0 0
*/

static int longest = 0;
static int R = 0;
static int C = 0;
static int m[102][102];
static int dp[102][102];
int dx[4] = {0, 0, 1, -1};
int dy[4] = {1, -1, 0, 0};

void ski(int x, int y) {
    if(dp[x][y]) return;
    int xx, yy;
    for(int i = 0; i < 4; i++) {
        xx = x + dx[i];
        yy = y + dy[i];
        if(xx >= 0 && yy >= 0 && xx <= R+1 && yy <=C+1 && m[x][y] > m[xx][yy]) {
            ski(xx, yy);
            dp[x][y] = max(dp[x][y], dp[xx][yy]+1);
        }
    }
}

int main () {
    cin>>R>>C;
    for (int i = 1; i <= R; i++) {
        for (int j = 1; j <= C; j++) {
            cin>>m[i][j];
        }
    }

    for (int i = 1; i <= R; i++) {
        for (int j = 1; j <= C; j++) {
            ski(i, j);
            longest = max(longest, dp[i][j]);
        }
    }

    /*cout<<endl;
    for (int i = 0; i <= R+1; i++) {
        for (int j = 0; j <= C+1; j++) {
            cout<<dp[i][j]<<' ';
        }
        cout<<endl;
    }*/

    cout<<longest;
    return 0;
}