#include <bits/stdc++.h>
#define syosu(x) fixed<<setprecision(x)
using namespace std;
typedef long long ll;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef pair<double,double> pdd;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<double> vd;
typedef vector<vd> vvd;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<string> vs;
typedef vector<P> vp;
typedef vector<vp> vvp;
typedef vector<pll> vpll;
typedef pair<int,P> pip;
typedef vector<pip> vip;
const int inf=1<<29;
const ll INF=1ll<<60;
const double pi=acos(-1);
const double eps=1e-11;
const ll mod=1e9+7;
const int dx[4]={-1,0,1,0},dy[4]={0,-1,0,1};

ll s,t,n;
vl a;

int main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	cin>>s>>t>>n;
	a=vl(n);
	ll sum=0,mn=0;
	for(int i=0;i<n;i++){
		cin>>a[i];
		sum+=a[i];
		mn=min(mn,sum);
	}
	ll d=s-t,res=(sum>=0?0:max(0LL,(d+mn)/(-sum)));
	d+=sum*res;
	res*=n;
	for(int i=0;i<3*n;i++){
		d+=a[i%n];
		if(d<=0){
			cout<<res+i+1<<endl;
			return 0;
		}
	}
	cout<<-1<<endl;
}
