import io
import sys

sys.stdin = io.StringIO('''3 6
10 5
9 2
8 1
''')  # 47

import math

n, m = map(int, input().split())
nums = []
for i in range(n):
    a, b = map(int, input().split())
    for j in range(math.ceil(a / b)):
        nums.append(a)
        a -= b

nums.sort(reverse=True)
print(sum(nums[:m]))