Uva 11661 - Burger Time?

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>

#define sf scanf
#define pf printf

using namespace std;

typedef long long LL;

char ch[2000000+5];

int main()
{
    LL t,i;
    while(~sf("%lld",&t) and t)
    {
        getchar();

        for(i=1;i<=t;i++)
        {
            sf("%c",&ch[i]);
        }

        bool f=false;
        LL dist_r=0, dist_d=0, mn=2000000000;

        for(i=1;i<=t;i++)
        {
            if(ch[i] == 'Z')
            {
                f=true;
                break;
            }
            else
            {
                if(ch[i] == 'R')
                {
                    dist_r=i;
                    if(dist_d)
                    {
                        mn = min(mn, abs(dist_r - dist_d));
                    }
                }
                if(ch[i] == 'D')
                {
                    dist_d = i;
                    if(dist_r)
                    {
                        mn = min(mn, abs(dist_r - dist_d));
                    }
                }
                //cout << mn << " ";
            }
        }

        if(f)
        {
            pf("0\n");
        }
        else
        {
            pf("%lld\n", mn);
        }
    }

    return 0;
}