import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


enum INF = 10L^^18;

// ceil(n/x) = k
// <=>  k-1 < n/x <= k
// <=>  n/k <= x < n/(k-1)
// <=>  ceil(n/k) <= x < ceil(n/(k-1))
// ceil(n/x) = floor((n-1)/x) + 1
void doCeils(alias fun, T)(const(T) n) {
  assert(n >= 1);
  for (T a = 0, b; a < n - 1; a = b) {
    const k = (n - 1) / (a + 1) + 1;
    b = (n - 1) / (k - 1);
    // (a, b]: k
    fun(a, b, k);
  }
  // n: 1
  fun(n - 1, n, 1);
}

void main() {
  try {
    for (; ; ) {
      const A = readLong;
      const B = readLong;
      const N = readLong;
      
      const K = cast(long)(sqrt(cast(real)(N)));
      const L = (N - 1) / K;
      auto small = new long[K + 1];
      auto large = new long[L + 1];
      long get(long k) {
        return (k <= K) ? small[k] : large[(N + k - 1) / k];
      }
      
      long calc(long k) {
        long ret = INF;
        if (k == 1) {
          ret = 0;
        } else {
          doCeils!((long a, long b, long kk) {
            if (kk < k) {
              chmin(ret, get(kk) + A + B * ((a + 1) - 1));
            }
          })(k);
        }
        debug {
          writeln(k, ": ", ret);
        }
        return ret;
      }
      foreach (k; 1 .. K + 1) {
        small[k] = calc(k);
      }
      foreach_reverse (l; 1 .. L + 1) {
        large[l] = calc((N + l - 1) / l);
      }
      
      const ans = get(N);
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}