Unique Paths

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?

C++:
int uniquePaths(int m, int n)
{
    int small=m<n?m:n;
    int large=m<n?n:m;
    double a=1;
    for(int i=1;i<small;i++)
         a=a*(small+large1i)/i;
    return a;
}
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