Which problem is similar with this problem?

Правка en4, от YeuDieuNhieu, 2018-05-25 07:09:38

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My solution: f[i] = f[i-1]+i with f[2]=4, but I'm not sure my solution is correct. Anyone here tell me which problem on any online judge similar with the beyond problem?

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en4 Английский YeuDieuNhieu 2018-05-25 07:09:38 4 Tiny change: '+i with f[1]=1, but I'm ' -> '+i with f[2]=4, but I'm '
en3 Английский YeuDieuNhieu 2018-05-25 07:08:49 8 Tiny change: '0"></a>)\nMy solut' -> '0"></a>)\n\n\n\n\nMy solut'
en2 Английский YeuDieuNhieu 2018-05-25 07:07:40 96
en1 Английский YeuDieuNhieu 2018-05-25 07:06:49 241 Initial revision (published)