n, d = map(int,input().split())
pq = []
for _ in range(n):
    p, q = map(int,input().split())
    pq.append((-p, q))

pq.sort(reverse = True)
ans = 0
now = 0
for i in range(d):
    now += pq[i % 2][0]
    ans = min(ans,now)
    now += pq[i % 2][1]
print(ans)