terminate called after throwing an instance of 'std::invalid_argument' what(): stoiclass Solution { public: string complexNumberMultiply(string a, string b) { int aplus = a. find('+'); int bplus = b. find('+'); int ai = a. find('i'); int bi = b. find('i'); int a1 = stoi(a. substr(0, aplus)); int b1 = stoi(a. substr(aplus + 1, ai), nullptr); int a2 = stoi(a. substr(0, bplus), nullptr); int b2 = stoi(a. substr(bplus + 1, bi), nullptr); int aa = a1 * a2 - (b1 * b2); int ab = a1 * b2 + a2 * b1; string res = to_string(aa) + "+" + to_string(ab) + "i"; return res; }};