/*  -*- coding: utf-8 -*-
 *
 * 2846.cc: Slimming Plan
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_D = 100000;
const long long LINF = 1LL << 62;

/* typedef */

typedef long long ll;

/* global variables */

ll wsums[MAX_D + 1];

/* subroutines */

inline void setmin(ll &a, ll b) { if (a > b) a = b; }
inline void setmax(ll &a, ll b) { if (a < b) a = b; }

/* main */

int main() {
  ll s, t;
  int d;
  scanf("%lld%lld%d", &s, &t, &d);
  ll dt = -(t - s);

  ll maxw = 0;
  for (int i = 0; i < d; i++) {
    int wi;
    scanf("%d", &wi);
    wsums[i + 1] = wsums[i] - wi;
    setmax(maxw, wsums[i + 1]);
  }
  //printf("dt=%lld, maxw=%lld, wsum=%lld\n", dt, maxw, wsums[d]);

  if (maxw >= dt) {
    int k = 0;
    for (; k <= d; k++)
      if (wsums[k] >= dt) break;
    printf("%d\n", k);
  }
  else if (wsums[d] > 0) {
    ll m = (dt - maxw + wsums[d] - 1) / wsums[d];
    dt -= m * wsums[d];
    int k = 0;
    for (; k <= d; k++)
      if (wsums[k] >= dt) break;
    printf("%lld\n", m * d + k);
  }
  else
    puts("-1");
  return 0;
}

