unique-path

A robot is located at the top-left corner of a m x n grid (marked ‘Start’ in the diagram below).
The robot can only move either down or right at any point in time. The robot is trying to reach the bottom-right corner of the grid (marked ‘Finish’ in the diagram below).
How many possible unique paths are there?

Above is a 3 x 7 grid. How many possible unique paths are there?
Note: m and n will be at most 100.

class Solution {
public:
    int uniquePaths(int m, int n) {
        if(m<=0||n<=0)
            return -1;
        vector<vector<int>> dp(m,vector<int>(n,0));
        for(int i=0;i<n;++i)
            {
            dp[0][i]=1;
        }
         for(int i=0;i<m;++i)
            {
            dp[i][0]=1;
        }
        for(int i=1;i<m;++i)
            {
            for(int j=1;j<n;++j)
                {
                dp[i][j]=dp[i-1][j]+dp[i][j-1];
            }
        }
        int res=dp[m-1][n-1];
        return res;
    }
};