import sys
sys.setrecursionlimit(5*10**5)
input = sys.stdin.readline
from collections import defaultdict, deque, Counter
from heapq import heappop, heappush
from bisect import bisect_left, bisect_right
from math import gcd

n,p,k = map(int,input().split())
tb = [list(map(int,input().split())) for i in range(n)]
dp = [[-1]*(k+1) for i in range(n+1)]
dp[0][0] = p
for i in range(1, n+1):
    t,b = tb[i-1]
    for j in range(k+1):
        if dp[i-1][j] != -1:
            dp[i][j] = dp[i-1][j]
        if j >= 1 and dp[i-1][j-1] != -1:
            dp[i][j] = min(10**18, max(dp[i][j], t*(dp[i-1][j-1]+b)))
ans = dp[-1][-1]
print(-1) if ans >= 10**18 else print(ans)