24 lines
665 B
C++
24 lines
665 B
C++
#include<cstdio>
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#include<algorithm>
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using namespace std;
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const int maxn=20005;
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int n,L[maxn],R[maxn],f[maxn][2];
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int main()
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{
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#ifdef local
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freopen("pro.in","r",stdin);
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#endif
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scanf("%d",&n);
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for(int i=1;i<=n;i++) scanf("%d%d",&L[i],&R[i]);
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f[1][0]=R[1]-1+R[1]-L[1]; f[1][1]=R[1]-1;
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for(int i=2;i<=n;i++)
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{
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f[i][0]=min(f[i-1][0]+abs(L[i-1]-R[i])+1+R[i]-L[i],f[i-1][1]+abs(R[i-1]-R[i])+1+R[i]-L[i]);
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f[i][1]=min(f[i-1][0]+abs(L[i-1]-L[i])+1+R[i]-L[i],f[i-1][1]+abs(R[i-1]-L[i])+1+R[i]-L[i]);
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}
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// for(int i=1;i<=n;i++)
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// printf("f[%d][0]=%d f[%d][1]=%d\n",i,f[i][0],i,f[i][1]);
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printf("%d\n",min(f[n][0]+abs(n-L[n]),f[n][1])+abs(n-R[n]));
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return 0;
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}
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