#include <bits/stdc++.h>

#define M_PI 3.14159265358979323846

using namespace std;

//typedef
//-------------------------#include <bits/stdc++.h>

#define M_PI 3.14159265358979323846

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s)
{
    int v;
    istringstream sin(s);
    sin >> v;
    return v;
}
template <class T>
inline string toString(T x)
{
    ostringstream sout;
    sout << x;
    return sout.str();
}
inline int readInt()
{
    int x;
    scanf("%d", &x);
    return x;
}

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<int, PII> TIII;
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;

//container util

//------------------------------------------
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a) * (a))
#define EACH(i, c) for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define EXIST(s, e) ((s).find(e) != (s).end())
#define SORT(c) sort((c).begin(), (c).end())

//repetition
//------------------------------------------
#define FOR(i, s, n) for (int i = s; i < (int)n; ++i)
#define REP(i, n) FOR(i, 0, n)
//#define MOD 1000000007

#define rep(i, a, b) for (int i = a; i < (b); ++i)
#define trav(a, x) for (auto &a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
const double EPS = 1E-8;

#define chmin(x, y) x = min(x, y)
#define chmax(x, y) x = max(x, y)
const int INF = 2147483647;

const int MAX = 510000;
const int MOD = 1000000007;

long long fac[MAX], finv[MAX], inv[MAX];

// テーブルを作る前処理
void COMinit()
{
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < MAX; i++)
    {
        fac[i] = fac[i - 1] * i % MOD;
        inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
        finv[i] = finv[i - 1] * inv[i] % MOD;
    }
}

// 二項係数計算
long long COM(int n, int k)
{
    if (n < k)
        return 0;
    if (n < 0 || k < 0)
        return 0;
    return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

ll S, T, D;
vector<ll> w(100010);
ll sum = 0;
ll mi = (1LL << 60);
bool C(ll x)
{
    ll ts = S + mi;
    ts += sum * (x / D);

    if (ts <= T)
        return true;
    else
        return false;
}
int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(15);

    cin >> S >> T >> D;
    for (int i = 0; i < D; i++)
    {
        cin >> w[i];
        sum += w[i];
        if (S + sum <= T)
        {
            cout << i + 1 << endl;
            return 0;
        }
        mi = min(mi, sum);
    }

    if (sum >= 0)
    {
        cout << -1 << endl;
        return 0;
    }
    else
    {

        ll lb = 0, ub = 100000000100;
        while (ub - lb > 1)
        {
            ll mid = (lb + ub) / 2;
            if (C(mid))
            {
                ub = mid;
            }
            else
            {
                lb = mid;
            }
        }
        ll ans = (1LL<<60);
        //cout << ub << endl;
        
        
        ll ts = S + sum * (max(0LL,(ub - D)) / D);
        //cout << ts << endl;
        for (ll i = 0; i < 2*D; i++)
        {
            ts += w[i%D];
            if (ts <= T)
            {
                ans = min(ans, ub - D + i + 1);
            }
        }
        cout << ans << endl;
    }

    return 0;
}



Measured memory_bytes: 3456