{"raw_statement":[{"iden":"problem statement","content":"There is a sequence $X$ of length $N$, where every element is initially $0$. Let $X_i$ denote the $i$\\-th element of $X$.\nYou are given a sequence $A$ of length $N$. The $i$\\-th element of $A$ is $A_i$. Determine if we can make $X$ equal to $A$ by repeating the operation below. If we can, find the minimum number of operations required.\n\n*   Choose an integer $i$ such that $1\\leq i\\leq N-1$. Replace the value of $X_{i+1}$ with the value of $X_i$ plus $1$."},{"iden":"constraints","content":"*   $1 \\leq N \\leq 2 \\times 10^5$\n*   $0 \\leq A_i \\leq 10^9(1\\leq i\\leq N)$\n*   All values in input are integers."},{"iden":"input","content":"Input is given from Standard Input in the following format:\n\n$N$\n$A_1$\n$:$\n$A_N$"},{"iden":"sample input 1","content":"4\n0\n1\n1\n2"},{"iden":"sample output 1","content":"3\n\nWe can make $X$ equal to $A$ as follows:\n\n*   Choose $i=2$. $X$ becomes $(0,0,1,0)$.\n*   Choose $i=1$. $X$ becomes $(0,1,1,0)$.\n*   Choose $i=3$. $X$ becomes $(0,1,1,2)$."},{"iden":"sample input 2","content":"3\n1\n2\n1"},{"iden":"sample output 2","content":"\\-1"},{"iden":"sample input 3","content":"9\n0\n1\n1\n0\n1\n2\n2\n1\n2"},{"iden":"sample output 3","content":"8"}],"translated_statement":null,"sample_group":[],"show_order":["default"],"formal_statement":null,"simple_statement":null,"has_page_source":true}