Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

func main() {
	io := NewIo()
	defer io.Flush()
	S, T, D := io.NextInt(), io.NextInt(), io.NextInt()
	w := make([]int, D)
	sum, min := 0, 0
	for i := 0; i < D; i++ {
		w[i] = io.NextInt()
		sum += w[i]
		min = intMin(sum, min)
		if S+sum <= T {
			io.Println(i + 1)
			return
		}
	}
	if sum >= 0 {
		io.Println(-1)
		return
	}
	ans := 0
	for ; S+min > T; ans += D {
		S += sum
	}
	for i := 0; i < D; i++ {
		S += w[i]
		if S <= T {
			io.Println(ans + i + 1)
			return
		}
	}
}

func intMin(a, b int) int {
	if a < b {
		return a
	}
	return b
}

// I/O template
type Io struct {
	reader    *bufio.Reader
	writer    *bufio.Writer
	tokens    []string
	nextToken int
}

func NewIo() *Io {
	return &Io{
		reader: bufio.NewReader(os.Stdin),
		writer: bufio.NewWriter(os.Stdout),
	}
}

func (io *Io) Flush() {
	err := io.writer.Flush()
	if err != nil {
		panic(err)
	}
}

func (io *Io) NextLine() string {
	var buff []byte
	for {
		line, isPrefix, err := io.reader.ReadLine()
		if err != nil {
			panic(err)
		}
		buff = append(buff, line...)
		if !isPrefix {
			break
		}
	}
	return string(buff)
}

func (io *Io) Next() string {
	for io.nextToken >= len(io.tokens) {
		line := io.NextLine()
		io.tokens = strings.Fields(line)
		io.nextToken = 0
	}
	s := io.tokens[io.nextToken]
	io.nextToken++
	return s
}

func (io *Io) NextInt() int {
	i, err := strconv.Atoi(io.Next())
	if err != nil {
		panic(err)
	}
	return i
}

func (io *Io) NextFloat() float64 {
	i, err := strconv.ParseFloat(io.Next(), 64)
	if err != nil {
		panic(err)
	}
	return i
}

func (io *Io) Println(a ...interface{}) {
	fmt.Fprintln(io.writer, a...)
}

func (io *Io) Printf(format string, a ...interface{}) {
	fmt.Fprintf(io.writer, format, a...)
}

func (io *Io) PrintIntSlice(a []int) {
	b := []interface{}{}
	for _, x := range a {
		b = append(b, x)
	}
	io.Println(b...)
}

func (io *Io) PrintStringSlice(a []string) {
	b := []interface{}{}
	for _, x := range a {
		b = append(b, x)
	}
	io.Println(b...)
}

func Log(name string, value interface{}) {
	fmt.Fprintf(os.Stderr, "%s=%+v\n", name, value)
}

func ExampleIo() {
	io := NewIo()
	defer io.Flush()
	a := make([]int, 0, 3)
	a[0], a[1], a[2] = io.NextInt(), io.NextInt(), io.NextInt()
	io.Println(a)
	io.PrintIntSlice(a)
	Log("io", a)
}

C++: #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;
}

C++: #include<iostream>
#include<string>
#include<cstdio>
#include<vector>
#include<cmath>
#include<algorithm>
#include<functional>
#include<iomanip>
#include<queue>
#include<ciso646>
#include<random>
#include<map>
#include<set>
#include<bitset>
#include<stack>
#include<unordered_map>
#include<utility>
#include<cassert>
using namespace std;

//#define int long long
typedef long long ll;

typedef unsigned long long ul;
typedef unsigned int ui;
const ll mod = 1000000007;
const ll INF = mod * mod;
typedef pair<int, int> P;
#define stop char nyaa;cin>>nyaa;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define Rep(i,sta,n) for(int i=sta;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define per1(i,n) for(int i=n;i>=1;i--)
#define Rep1(i,sta,n) for(int i=sta;i<=n;i++)
typedef pair<ll, ll> LP;
typedef vector<ll> vec;
typedef long double ld;
typedef pair<ld, ld> LDP;
const ld eps = 1e-5;

void solve(){
	int s, t, d;
	cin >> s >>t >> d;
	vector<ll> w(d);
	rep(i, d) {
		cin >> w[i];
	}
	ll mi = 0;
	ll r = 0;
	rep(i, d) {
		r += w[i]; mi = min(mi, r);
		if (r + s <= t) {
			cout << i + 1 << endl; return;
		}
	}
	if (r >= 0) {
		cout << -1 << endl; return;
	}
	ll dif = (s - t + mi + abs(r) - 1) / abs(r);
	ll ans = dif * d;
	s -= dif * abs(r);
	if (s <= t) {
		cout << ans << endl; return;
	}
	rep(i, d) {
		ans++;
		s += w[i];
		if (s <= t) {
			cout << ans << endl; return;
		}
	}
}
signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	//cout << fixed << setprecision(5);
	//init();
	solve();

	//cout << "finish" << endl;
	//stop
	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int main(){
  ll s,t,d,w[100000],c=0;
  cin>>s>>t>>d;
  s-=t;
  for(int i=0;i<d;i++)cin>>w[i],c+=w[i];
  ll ans=0,tp=s;
  for(int i=0;tp>0&&i<d;i++)tp+=w[i],ans++;
  if(tp>0){
    if(c<0){
      ans=(s/-c-1)*d;
      s=s%-c+d;
      for(int i=0;s>0;i++)s+=w[i],ans++;
    }
    else ans=-1;
  }
  cout<<ans<<endl;
  return 0;
}