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

L.sort(key=lambda x: (x[0], -x[2]))
ans = 10**10
curr = 0
for i in range(d):
    curr -= L[i][0]
    ans = min(ans, curr)
    curr += L[i][1]
    ans = min(ans, curr)
print(ans)