25 lines
324 B
C
25 lines
324 B
C
#include <stdio.h>
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int main()
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{
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long n, i;
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long unsigned_a[201] = {};
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long *a = &unsigned_a[100];
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char a0;
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long long v;
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scanf("%ld", &n);
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for (i = 0; i < n; i++) {
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scanf("%hhd", &a0);
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a[a0]++;
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}
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v = a[0] * (a[0] - 1) / 2;
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for (i = 1; i <= 100; i++)
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v += a[i] * a[-i];
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printf("%lld\n", v);
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return 0;
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}
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