import sys def input(): return sys.stdin.readline().strip() def list2d(a, b, c): return [[c] * b for i in range(a)] def list3d(a, b, c, d): return [[[d] * c for j in range(b)] for i in range(a)] def list4d(a, b, c, d, e): return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)] def ceil(x, y=1): return int(-(-x // y)) def INT(): return int(input()) def MAP(): return map(int, input().split()) def LIST(N=None): return list(MAP()) if N is None else [INT() for i in range(N)] def Yes(): print('Yes') def No(): print('No') def YES(): print('YES') def NO(): print('NO') sys.setrecursionlimit(10 ** 9) INF = 10 ** 18 MOD = 10 ** 9 + 7 EPS = 10 ** -10 N, D = MAP() TK = [] for i in range(N): t, k = MAP() TK.append((t, k)) dp0 = [-INF] * (N+1) dp1 = [-INF] * (N+1) dp0[0] = 0 for i, (t, k) in enumerate(TK): dp0[i+1] = max(dp0[i+1], dp0[i] + t) dp0[i+1] = max(dp0[i+1], dp1[i] + t - D) dp1[i+1] = max(dp1[i+1], dp1[i] + k) dp1[i+1] = max(dp1[i+1], dp0[i] + k - D) ans = max(dp0[N], dp1[N]) print(ans)