cc1plus: error: '::main' must return 'int'
tower.cpp: In function 'int main()':
tower.cpp:10:8: error: 'a' was not declared in this scope
10 | sort(a.begin(), a.end());
| ^
tower.cpp:13:44: error: 'd' was not declared in this scope
13 | while (p < N - 1 && a[p + 1] - a[i] <= d) p++;
| ^
tower.cpp:15:9: warning: operation on 'ans' may be undefined [-Wsequence-point]
15 | ans = (ans = ans * d) % mod;
| ~~~~^~~~~~~~~~~~~~~~~~~~~~~
tower.cpp:15:9: warning: operation on 'ans' may be undefined [-Wsequence-point]