#include<bits/stdc++.h> #define rep(i,a,b) for(int i=a;i<b;i++) #define rrep(i,a,b) for(int i=a;i>=b;i--) #define fore(i,a) for(auto &i:a) #pragma GCC optimize ("-O3") using namespace std; void _main(); int main() { cin.tie(0); ios::sync_with_stdio(false); _main(); } //--------------------------------------------------------------------------------------------------- /*--------------------------------------------------------------------------------------------------- ∧_∧ ∧_∧ (´<_` ) Welcome to My Coding Space! ( ´_ゝ`) / ⌒i / \ | | / / ̄ ̄ ̄ ̄/ | __(__ニつ/ _/ .| .|____ \/____/ (u ⊃ ---------------------------------------------------------------------------------------------------*/ typedef long long ll; #define INF 1LL<<60 int N; ll D, C[101][2]; ll dp[101][2]; // dp[day][0=tokyo|1=kyoto] //--------------------------------------------------------------------------------------------------- void _main() { cin >> N >> D; rep(i, 0, N) cin >> C[i][0] >> C[i][1]; rep(i, 0, N + 1) rep(j, 0, 2) dp[i][j] = -INF; dp[0][0] = 0; dp[0][1] = -D; rep(d, 0, N) rep(p, 0, 2) { dp[d + 1][p] = max(dp[d + 1][p], dp[d][p] + C[d][p]); dp[d + 1][1 - p] = max(dp[d + 1][1 - p], dp[d][p] + C[d][1 - p] - D); } ll ans = max(dp[N][0], dp[N][1]); cout << ans << endl; }